Prenatal Development Timeline

Nervous Nervous Cardiovascular Cardiovascular Muscular Muscular Early Events Early Events
Special Senses Special Senses Respiratory Respiratory Skeletal Skeletal Growth Parameters Growth Parameters
Blood & Immune Blood & Immune Gastrointestinal Gastrointestinal Endocrine Endocrine General General
Skin/Integument Skin/Integument Renal/Urinary Renal/Urinary Reproductive Reproductive Movement Movement

Unit 1:   The First Week 
Day 0Early Events Embryonic period begins
Day 1Early Events Embryo is spherically shaped and called a morula comprised of 12 to 16 blastomeres
  Early Events Embryo is spherically shaped with 12 to 16 cells
  Early Events Fertilization resulting in zygote formation
Day 1 - Day 1Early Events Fertilization - development begins with a single-cell embryo!!!
Day 2Endocrine Early pregnancy factor (EPF)
  Early Events Activation of the genome
  Early Events Blastomeres begin rapidly dividing
  Early Events Zygote divides into two blastomeres (24 – 30 hours from start of fertilization)
Day 3Early Events Compaction
Day 4Early Events Embryonic disc
  Early Events Free floating blastocyst
  Early Events Hypoblast & epiblast
  Early Events Inner cell mass
  Early Events See where the back and chest will be
Day 5Early Events Hatching blastocyst
Day 6Early Events Embryo attaches to wall of uterus
  Early Events Solid synctiotrophoblast & cytotrophoblast
1 weekEarly Events Chorion
  Early Events Chorionic cavity
  Early Events Extra-embryonic mesoderm (or mesoblast)
  Early Events Placenta begins to form
Unit 2:   1 to 2 Weeks 
1 week, 1 dayEndocrine Positive pregnancy test
  Early Events Amnioblasts present; amnion and amniotic cavity formation begins
  Early Events Bilaminar embryonic disc
1 week, 2 daysEndocrine Corpus luteum of pregnancy
  Early Events Cells in womb engorged with nutrients
  Early Events Exocoelomic membrane
  Early Events Isolated trophoblastic lacunae
  General Embryonic disc 0.1 mm diameter
1 week, 4 daysEarly Events Intercommunicating lacunae network
  Early Events Longitudinal axis
  Early Events Prechordal plate
  Early Events Trophoblastic vascular circle
1 week, 5 daysEarly Events Implantation complete
  Early Events Yolk sac
  General Embryonic disc diameter: 0.15 to 0.20 mm
1 week, 6 daysBlood & Immune Blood islands in umbilical vesicle
  Renal/Urinary Allantois
  Cardiovascular Angiogenesis in chorionic mesoblast
  Cardiovascular Blood vessels in villi
  Cardiovascular Connecting stalk
  Cardiovascular Primordial blood vessels
  Early Events Amnion with single cell layer
  Early Events Chorionic villi
2 weeksEarly Events Embryonic epiblast gives rise to primitive streak and primitive node and
  Early Events Yolk sac
  Early Events Yolk sac
Unit 3:   2 to 3 Weeks 
2 weeks, 1 dayEarly Events 3 germ layers
  Early Events Cloacal membrane
  Early Events Primitive groove
  Early Events Rostral-caudal orientation
2 weeks, 2 daysBlood & Immune Erythroblasts in yolk sac
  Blood & Immune Three types of blood-forming cells in yolk sac
  Reproductive Primordial germ cells
  Renal/Urinary Allantoic diverticulum
  Early Events Allantoic diverticulum
  Early Events Amnion with two cell layers
  Early Events Notochordal process
  Early Events Secondary villi
2 weeks, 4 daysGastrointestinal Foregut, midgut, and hindgut
  Cardiovascular Uteroplacental circulation well established
  Cardiovascular Prechordal plate with 1 retinal field
  Special Senses Brain is first organ to appear
  Special Senses Caudal eminence
  Special Senses Neural ectoderm
  Special Senses Neural groove and neural folds
  Special Senses Neural plate induced by notochordal process
  Special Senses Notochordal and neurenteric canals
  Special Senses Notochordal plate
  Early Events Connecting stalk
  Early Events Primitive pit (or notochordal pit)
2 weeks, 5 daysEarly Events Prechordal plate with 2 retinal fields
2 weeks, 6 daysBlood & Immune Numerous blood islands in umbilical vesicle
  Muscular Septum transversum (primitive diaphragm)
  Gastrointestinal Foregut
  Gastrointestinal Oropharyngeal membrane
  Gastrointestinal Pharyngeal pouch 1
  Gastrointestinal Stomodeum forming
  Cardiovascular Beginnings of the heart can be seen
  Cardiovascular Blood vessels emerge simultaneously in umbilical vesicle, embryo proper, amnion, and connecting stalk
  Cardiovascular Common umbilical artery
  Cardiovascular Dorsal aortae (paired)
  Cardiovascular First pair of aortic arches
  Cardiovascular Heart: Cardiogenic plate, cardiac jelly, myocardial mantle, and endocardial plexus
  Cardiovascular Left ventricle, right ventricle, conotruncus
  Cardiovascular Paired pericardial cavities
  Cardiovascular Paired tubular heart
  Special Senses Forebrain, midbrain, and hindbrain
  Special Senses Hindbrain with four rhombomeres
  Special Senses Isthmus rhombencephali demarcates midbrain and hindbrain
  Special Senses Mesencephalon (or midbrain)
  Special Senses Neural cord within caudal eminence
  Special Senses Neural groove deepens substantially
  Special Senses Primary neuromeres
  Special Senses Three main divisions of brain
  Early Events Cephalic and caudal folds
  Early Events Neural crest: Rostral and facial
  Early Events Primitive streak reaches neurenteric canal
  Early Events Somites with central somitocoels: Pairs 1 through 3
3 weeksCardiovascular Blood and blood vessels
Unit 4:   3 to 4 Weeks 
3 weeks, 1 dayEndocrine Thyroid primordium emerges from floor of pharynx
  Renal/Urinary Nephrogenic cord emerges (at 10 somites)
  Gastrointestinal Cloaca
  Gastrointestinal Common coelomic cavity divides into peritoneal, pericardial, and pleural cavities
  Gastrointestinal Liver: Hepatic plate (endoderm)
  Gastrointestinal Midgut emerging
  Gastrointestinal Pharyngeal arches 1 and 2
  Gastrointestinal Pharyngeal cleft 1
  Gastrointestinal Second pharyngeal cleft and pouch
  Respiratory Pharyngeal groove and ridge with laryngotracheal sulcus
  Respiratory Respiratory outgrowth
  Cardiovascular Atria (right and left) far apart
  Cardiovascular Bulbis cordis
  Cardiovascular Circulatory system function begins
  Cardiovascular Endocardial tubes fuse forming tubular heart
  Cardiovascular Heart begins beating
  Cardiovascular Pericardial sac
  Cardiovascular Pericardium
  Cardiovascular Primary head vein
  Cardiovascular Sinus venosus
  Cardiovascular Tubular heart begins folding
  Cardiovascular Umbilical arteries
  Cardiovascular Umbilical veins (right and left)
  Nervous Optic primordia fill neuromere D2
  Nervous Otic pits
  Special Senses Chiasmatic plate
  Special Senses Mesencephalic flexure
  Special Senses Neural tube
  Special Senses Neuromeres D1 and D2 (in diencephalon)
  Special Senses Optic sulcus in forebrain
  Special Senses Pontine region identifiable near cranial nerves VII and VIII
  Special Senses Segment D in rhombencephalon
  Special Senses Some secondary neuromeres
  Special Senses Superior colliculus
  Special Senses Telencephalon
  Special Senses Telencephalon (or telencephalic) medium
  Early Events Body cavities
  Early Events Hyoid arch
  Early Events Mandibular arch and maxillary process
  Early Events Neural crest: Trigeminal, facioacoustic, glossopharyngeal-vagal, and occipitospinal
  Early Events Somites: Pairs 4 through 12
3 weeks, 3 daysReproductive Primordial germ cells begin moving from umbilical vesicle to hindgut
  Endocrine Thyroid complete
  Skeletal Face: Maxillary and mandibular processes (bilaterally)
  Renal/Urinary Cloacal membrane
  Renal/Urinary Mesonephric duct emerges from nephrogenic cord
  Renal/Urinary Nephric vesicles
  Gastrointestinal Cystic primordium
  Gastrointestinal Hepatic diverticulum
  Gastrointestinal Liver
  Gastrointestinal Membrane between future mouth and throat may begin to rupture
  Cardiovascular Angiogenesis along surface of central nervous system
  Cardiovascular Aortic sac
  Cardiovascular Atrioventricular canal
  Cardiovascular Capillary plexus begins forming around brain and spinal cord
  Cardiovascular Conotruncus
  Cardiovascular Conus cordis emerging from right ventricle
  Cardiovascular Endocardium
  Cardiovascular Heart contractions produce peristaltic blood flow
  Cardiovascular Internal carotid arteries
  Cardiovascular Interventricular septum
  Cardiovascular Primordium of myocardium
  Cardiovascular Sinus venosus separating from left atria
  Cardiovascular Trabeculated outpouches along primary cardiac tube representing primordia of left and right ventricles
  Cardiovascular Trigeminal and otic arteries
  Nervous Facio-vestibulocochlear ganglia (CN VII, CN VIII)
  Nervous Glossopharyngeal and vagal ganglia
  Nervous Optic evagination (starting at 14 somites)
  Nervous Otic vesicle
  Nervous Trigeminal ganglia (CN V)
  Nervous Neural crest: Optic crest emerges during Carnegie Stages 11 and 12
  Nervous Nose: Nasal plate
  Nervous Optic vesicles form (17 to 19 somites)
  Special Senses Adenohypophysial pouch
  Special Senses Adenohypophysis
  Special Senses Lamina terminalis
  Special Senses Mesencephalon contains tectum and tegmentum
  Special Senses Neural crest production and migration continue
  Special Senses Neurohypophysial primordia
  Special Senses Neuropore (near brain) closes
  Special Senses Notochord
  Early Events Segmentation of mesoblast alongside neural tube bilaterally
  Early Events Somites: Pairs 13 through 20
3 weeks, 3 days - 5 weeks, 6 daysSpecial Senses All eight rhombomeres (Rh 1 through Rh 7, Rh D) - Present in stages 11 through 17
3 weeks, 5 daysEndocrine Telopharyngeal bodies
  Gastrointestinal Alimentary epithelium invades stroma of liver
  Gastrointestinal Alimentary epthelium proliferates in primordia of stomach, liver, and dorsal pancreas
  Gastrointestinal First part of pancreas
  Gastrointestinal Gastric portion of foregut elongates (25 to 28 somites)
  Gastrointestinal Hepatic primordium with abundant vascular plexus
  Gastrointestinal Omental bursa
  Gastrointestinal Oropharyngeal membrane is ruptured
  Gastrointestinal Pharyngeal arch 3
  Gastrointestinal Pharyngeal arches with dorsal and ventral parts
  Gastrointestinal Umbilical vesicle elongates
  Respiratory Cervical sinus
  Respiratory Laryngotracheal groove
  Respiratory Lung bud
  Respiratory Tracheo-esophageal septum
  Cardiovascular Atrioventricular canal
  Cardiovascular Common cardinal veins (right and left)
  Cardiovascular Descending aorta
  Cardiovascular Heart circulates blood to and from central nervous system, umbilical vesicle, and chorion
  Cardiovascular Hepatocardiac channels (right and left)
  Cardiovascular Rostral and caudal cardinal veins along brain and spinal cord feeding common cardinal veins
  Cardiovascular Septum primum and foramen primum sometimes present
  Cardiovascular Sinu-atrial foramen prevents backflow into sinus venosus
  Cardiovascular Sinus venosus collects veinous blood from entire embryo
  Cardiovascular Superior vena cava, inferior vena cava, and sinus venosus collecting all venous blood
  Cardiovascular Unidirectional circulation
  Cardiovascular Vitelline arteries and veins
  Nervous Hypoglossal cord (CN XII) enters pharyngeal arch 4
  Nervous Otocyst nearly closed
  Nervous Nasal discs form part of ectodermal ring
  Nervous Optic vesicles covered by sheath (formed by mesencephalic and optic crest)
  Special Senses Brain involves 40% of neural tube
  Special Senses Brain: Embryonic commissural plate
  Special Senses Ectodermal ring complete
  Special Senses Hypoglossal nucleus (CN XII)
  Special Senses Lowermost spinal cord formation begins
  Special Senses Mamillary recess
  Special Senses Marginal layer in rhombencephalon
  Special Senses Mesencephalic flexure at 90 degrees
  Special Senses Mesencephalon with two neuromeres: M1 and M2
  Special Senses Motor neurons in basal plate of rhombencephalon
  Special Senses Neural tube closes (lower back)
  Special Senses Neurofibrils form in rhombencephalon
  Special Senses Primary neurulation ends
  Special Senses Primordia of ventral thalamus and subthalamus in diencephalon
  Special Senses Sulcus limitans
  Special Senses Sulcus limitans in midbrain
  Early Events Somites: Pairs 21 through 29
  Early Events Upper limb primordium at level of somites 8 to 10
  General Progressively C-shaped embryo
4 weeksBlood & Immune Spleen primordia
  Blood & Immune Thymic primordia
  Skin Lower lip forms from merging of mandibular processes
  Skin Melanoblasts in epidermis
  Skin Skin is so thin, you can see through it!
  Reproductive Gonadal ridge extends from C-7 to T-8 levels
  Reproductive Primordial germ cells migrate to mesonephric ridges
  Reproductive Primordial germ cells number several hundred
  Reproductive Urorectal septum
  Endocrine Thyroid bilobed and attached to pharynx by thryoglossal duct
  Muscular Diaphragm primordia
  Renal/Urinary Glomeruli emerge in mesonephros
  Renal/Urinary Mesonephric duct attached to cloaca
  Renal/Urinary Nephric tubules now S-shaped
  Renal/Urinary Urogenital sinus
  Renal/Urinary Urorectal cleavage line
  Gastrointestinal Diverticulum ilei marks division between foregut and hindgut
  Gastrointestinal Esophagus primordia
  Gastrointestinal Intestines growing in length
  Gastrointestinal Mesentery from end of duodenum to proximal half of colon
  Gastrointestinal Opening between gut and umbilical vesicle decreases
  Gastrointestinal Pancreas: Ventral pancreas
  Gastrointestinal Pharyngeal pouches 1 through 4
  Gastrointestinal Pharynx
  Gastrointestinal Pleuroperitoneal canals
  Gastrointestinal Small & large intestines
  Gastrointestinal Stalk of umbilical vesicle lengthens and narrows
  Gastrointestinal Stomach assumes shape of a spindle
  Gastrointestinal Umbilical vesicle at height of development
  Gastrointestinal Vitelline duct
  Respiratory Bronchial buds
  Respiratory Lungs begin filling chest cavity
  Respiratory Mesenchyme from coelomic epithelium surrounds esophagus and lung buds
  Respiratory Trachea
  Cardiovascular Anterior, middle, and posterior cerebral plexuses
  Cardiovascular Aorta branches include dorsal intersegmental, lateral segmental, and ventral segmental arteries
  Cardiovascular Aortic arches 4 and 6
  Cardiovascular Artery from the common iliac artery feeds each lower limb bud
  Cardiovascular Atrioventricular bundle
  Cardiovascular Cardiac contractions still under myogenic control
  Cardiovascular Celiac artery, superior and inferior mesenteric arteries
  Cardiovascular Circulatory system "well established"
  Cardiovascular Common iliac arteries (right and left, from dorsal aorta bifurcation)
  Cardiovascular Contractions well coordinated and sequential from sinus venosus to atria to ventricles
  Cardiovascular Ductus venosus
  Cardiovascular Functioning two-chamber heart
  Cardiovascular Gas exchange through placenta begins
  Cardiovascular Gelatinous reticulum (or cardiac mesenchyme)
  Cardiovascular Heart chambers bulging with fluid
  Cardiovascular Heart now functions as two parallel pumps
  Cardiovascular Heart rate (about) 113 beats/min
  Cardiovascular Heart: Atrioventricular cushions (rostroventral and caudodorsal)
  Cardiovascular Heart: Myocardium wall 3 to 4 cells thick
  Cardiovascular Liver: Hepatocardiac vein
  Cardiovascular Primary head veins (right and left) drain anterior, middle, and posterior cerebral plexuses and feed precardinal veins
  Cardiovascular Septum primum, foramen primum
  Cardiovascular Small arteries emerging throughout mesoderm
  Cardiovascular Ventricle walls trabeculated
  Cardiovascular Vertebral arteries
  Cardiovascular Vitelline veins empty exclusively into hepatic plexus
  Nervous Most cranial nerve ganglia
  Nervous Trigeminal, glossopharyngeal, and vagal preganglia
  Nervous Basement membrane of otic disc surrounds otic vesicle
  Nervous Endolymphatic appendage
  Nervous Otic invagination
  Nervous Otic vesicle closes
  Special Senses Brain: Commissural plate
  Special Senses Cerebellum
  Special Senses Common afferent tract
  Special Senses Fourth ventricle
  Special Senses Interstitial nucleus (part of medial longitudinal fasciculus)
  Special Senses Isthmus rhombencephali (a new neuromere)
  Special Senses Oculomotor (CN III) and trochlear nuclei (CN IV) in mesencephalon (midbrain) and isthmus respectively
  Special Senses Retinal and lens discs
  Special Senses Terminal-vomeronasal neural crest
  Early Events Amnion surrounds connecting stalk and vitelline stalk
  Early Events Amnion surrounds embryo
  Early Events Cervical flexure
  Early Events Hyoid arch sudivides into dorsal and ventral segments
  Early Events Limb buds - the first sign of arms and legs
  Early Events Lower limb buds
  Early Events Umbilical cord emerging
  Early Events Upper and lower limb buds
Unit 5:   4 to 5 Weeks 
4 weeks, 3 daysEarly Events Early eyes
4 weeks, 3 days - 5 weeksReproductive Germ cells migrate to gonads
4 weeks, 4 daysBlood & Immune Thymus
  Endocrine Parathyrogenic zones
  Endocrine Thyroglossal duct
  Endocrine Thyroid pedical lengthens
  Skeletal Dorsal contour develops depression at level of sclerotomes 4 and 5
  Muscular Muscular plates between upper and lower limb buds
  Renal/Urinary Glomerular capsules, partially vascularized
  Renal/Urinary Mesonephric corpuscle
  Renal/Urinary Metanephrogenic cap emerges from ureteric bud
  Renal/Urinary Ureteric buds
  Gastrointestinal Angiogenesis within peri-esophageal mesenchyme
  Gastrointestinal Epiploic foramen
  Gastrointestinal Lesser sac (omental bursa)
  Gastrointestinal Small intestine forming coils
  Gastrointestinal Tongue: Hypopharyngeal eminence
  Respiratory Arytenoid swellings (right and left)
  Respiratory Capillary network surrounds pulmonary mesenchyme
  Respiratory Epithelial lamina of larynx
  Respiratory Lungs: Right and left primary (or main stem) bronchi
  Respiratory Mesenchyme covering esophagus and respiratory tree separates
  Respiratory Mesenchyme surrounds bronchi
  Respiratory Pleura (mesothelium) surrounds part of mesenchyme
  Respiratory Right main bronchus longer than left
  Cardiovascular Atria walls thin, ventricle walls thick and trabeculated
  Cardiovascular Atrioventricula cushions not fused
  Cardiovascular Common pulmonary vein drains pulmonary plexuses into left atrium
  Cardiovascular Conotruncal ridges or cushions (remnants of cardiac jelly)
  Cardiovascular Epicardium
  Cardiovascular Outflow tract still with one lumen
  Cardiovascular Posterior communicating arteries
  Cardiovascular Pulmonary arch (sixth aortic arch) forms from aorta and aortic sac
  Cardiovascular Pulmonary capillary network fed by pulmonary arteries, drain into left atrium
  Cardiovascular Sinu-atrial (SA) node
  Cardiovascular Superior mesenteric artery and vein
  Cardiovascular Upper limb buds with early marginal blood vessel
  Nervous Brachial plexus
  Nervous Cervical plexus
  Nervous Dorsal roots
  Nervous Hypoglossal nerve roots unite (CN XII)
  Nervous Lens and retina invaginate to form optic cup
  Nervous Primordium of cochlear duct
  Nervous Rami communicantes
  Nervous Spinal nerves reach muscle primordia
  Nervous Upper limb buds innervated
  Nervous External ear: Auricular hillocks merging
  Nervous Eyes located on sides of head
  Nervous Lens pits
  Nervous Lens vesicle open to surface (lens pore)
  Nervous Nose: Nasal pits
  Nervous Nose: Nasal plate (or disc) flat or concave
  Nervous Pigment in retina (external layer of optic cup)
  Special Senses D1 and D2 no longer identifiable within diencephalon
  Special Senses 75% of midbrain covered by marginal layer
  Special Senses All 16 secondary neuromeres
  Special Senses Brain enlarges 50% since Carnegie Stage 13
  Special Senses Brain: Cerebral hemispheres appear and begin rapid growth
  Special Senses Brain: Lateral ventricles
  Special Senses Cerebellum with intermediate and ventricular layers
  Special Senses Cerebellum: Primordium found in alar plate of rhombomere 1
  Special Senses Corpora striata primordia connected by commissural plate
  Special Senses Cranial nerve 3
  Special Senses Di-telencephalic sulcus
  Special Senses Dorsal and ventral thalami
  Special Senses Dorsal funiculus
  Special Senses Hypothalamic sulcus
  Special Senses Hypothalamus
  Special Senses Mamillary region
  Special Senses Medial and lateral longitudinal fasciculi
  Special Senses Median ventricular eminence
  Special Senses Pontine flexure
  Special Senses Preoptic sulcus extends between optic evaginations
  Special Senses Preoptico-hypothalamo-tegmental tract
  Special Senses Primary meninx surrounds most of brain
  Special Senses Rhombic lip
  Special Senses Spinal cord wall with three zones: ventricular (ependymal) zone, mantle (intermediate) zone, and marginal zone
  Special Senses Subthalamus with medial striatal ridge emerging
  Special Senses Synencephalon
  Special Senses Tegmentum
  Special Senses Tentorium cerebelli, medial portion
  Special Senses Terminal-vomeronasal crest contacts brain (olfactory area)
  Special Senses Torus hemisphericus (TH)
  Special Senses Velum transversum
  Special Senses Ventral longitudinal fasciculus
  Early Events Ventral segment of hyoid arch subdivides
4 weeks, 5 daysSkin Primordium of antitragus emerges from ventral subsegment of hyoid arch
  Reproductive Gonad framework found in coelomic epithelium
  Endocrine Thyroid detached from epithelium of pharynx in some embryos
  Skeletal Lower limb bud rounded proximally and tapered distally
  Skeletal Mesenchymal skeleton in upper and lower limbs
  Skeletal Right and left neural processes
  Skeletal Sclerotomic material around notochord (rhombomere D level)
  Skeletal Vertebrae well defined
  Skeletal Vertebral centra
  Renal/Urinary Primary urogenital sinus
  Renal/Urinary Ureteric bud extends to pelvis of the ureter
  Gastrointestinal Bladder and rectum are separating caudal to ureters
  Gastrointestinal Caecum
  Gastrointestinal Dense mesenchyme surrounds much of gastrointestinal tract
  Gastrointestinal Esophagus elongates, passes dorsal to carina and between main stem bronchi
  Gastrointestinal Gall bladder and cystic duct
  Gastrointestinal Liver: Hepatic ducts
  Gastrointestinal Ventral pancreas appears as an offshoot of the cystic duct
  Respiratory Lobar bud swellings denote areas of secondary bronchi
  Respiratory Remnants of coelomic epithelium forming visceral pleura
  Cardiovascular Atrioventricular cushions apposed
  Cardiovascular Blood flow divided into right and left streams through atrioventricular canal, ventricles, outflow tract, and aortic sac
  Cardiovascular Blood vessels penetrate diencephalon
  Cardiovascular Capillary plexus surrounds esophagus
  Cardiovascular Capillary plexus surrounds lung buds
  Cardiovascular Cardiac mesenchyme surrounds ventricles and outflow tract
  Cardiovascular Coronary arteries (terminal end)
  Cardiovascular Foramen secundum begins in septum primum
  Cardiovascular Left ventricle with thicker walls and greater volume than right
  Cardiovascular Semilunar cusps
  Nervous Capsule present around lens
  Nervous Corneal epithelium overlying optic cup
  Nervous Ear: Endolymphatic duct
  Nervous Geniculate and vestibulocochlear ganglia separating
  Nervous Lens body now present containing some lens fibers
  Nervous Lower limb buds innervated
  Nervous Optic stalk
  Nervous Utricle, endolymphatic duct, and endolymphatic sac
  Nervous Utriculo-endolymphatic fold
  Nervous External ear primordia emerges from caudolateral portion of mandibular arch
  Nervous Face: Lateral and medial nasal processes bilaterally
  Nervous Lateral nasal processes along dorsolateral lip of nasal pits
  Nervous Lens vesicles closed, pores absent
  Nervous Nose: Nasal discs recede forming nasal pits
  Nervous Optic chiasm
  Special Senses Adult lamina terminalis
  Special Senses Amygdaloid area
  Special Senses Brain with five main sections
  Special Senses Cerebellar plate
  Special Senses Cerebellum with marginal layer
  Special Senses Fibers of dorsal funiculus reach level of C1
  Special Senses First axodendritic synapses in cervical spinal cord
  Special Senses First nerve fibers
  Special Senses Habenular nucleus
  Special Senses Habenulo-interpeduncular tract
  Special Senses Lateral striatal ridge (derived from telencephalon and comprised mainly of neostriatum)
  Special Senses Lateral ventricular eminence
  Special Senses Locus caeruleus
  Special Senses Longitudinal zones in diencephalon
  Special Senses Marginal layer throughout most of diencephalon
  Special Senses Material for sympathetic trunks scattered in cervical region
  Special Senses Median striatal ridge (paleostriatum)
  Special Senses Mesencephalic tract of CN 5
  Special Senses Most cranial nerves seen
  Special Senses Olfactory fibers reach brain
  Special Senses Optic groove (also called preoptic recess)
  Special Senses Postoptic recess
  Special Senses Primordium of epiphysis
  Special Senses Rhombomeres still identifiable
  Special Senses Superior colliculi and its commissure
  Special Senses Superior medullary velum
  Special Senses Supramamillary commissure
  Special Senses Synapses among motor neurons in spinal cord
  Special Senses Tectobulbar tract
  Special Senses Tentorium
  Special Senses Third ventricle
  Special Senses Trigemino-cerebellar tract
  Special Senses Trochlear nerve root and decussation (CN IV)
  Early Events Hand plate emerges from distal upper limb bud
  General Frontonasal prominence
5 weeksEndocrine ACTH [adrenocorticotropin hormone]
  Endocrine Growth hormone
  Endocrine Pituitary gland
  Skeletal Limb buds form hand plates
  Renal/Urinary Permanent kidneys
  Respiratory Arytenoid and epiglottal swellings
  Respiratory Bronchial tree branching accelerates
  Respiratory Lobar pattern mimics adult pattern
  Respiratory T-shaped laryngeal inlet
  Cardiovascular Pacemaker cells
  General Head is one third of entire embryo
Unit 6:   5 to 6 Weeks 
5 weeks, 1 daySkeletal Wrist joints are forming
5 weeks, 2 daysSkin Apical epidermal ridges
  Skin Mammary ridge
  Skin Maxillary and premaxillary fields still widely separated
  Skin Nipples emerge from mammary crest
  Reproductive Gonad region separates from mesonephros
  Reproductive Gonadal primordium
  Reproductive Labioscrotal swelling
  Reproductive Urogenital fold and groove
  Endocrine Suprarenal gland: Cortex primordium
  Endocrine Suprarenal gland: Medulla
  Endocrine Thyroid detaches from pharynx
  Endocrine Thyroid with right and left lobes connected by an isthmus
  Skeletal Cartilage in mandibular arch
  Skeletal Hand area with central carpal region and digital plate with marginal vein
  Skeletal Pre-chondrocranium: Otic capsule, nasal capsule, and parachordal condensations
  Skeletal Primordia of primary palate
  Skeletal Ribs: Primordia now present for all 12 pairs
  Skeletal Vertebral column with 36 levels of ganglia and myotomes
  Muscular Extra-ocular premuscle masses receive cranial nerve fibers [oculomotor (CN III), trochlear (CN IV), and abducens (CN VI) nerves]
  Muscular Gluteal mesoderm
  Muscular Infrahyoid premuscle masses
  Muscular Limb mesoderm
  Muscular Sternocleidomastoid-trapezius premuscle mass with spinal accessory nerve (CN11)
  Muscular Thigh and thigh mesoderm
  Muscular Tongue premuscle mass
  Renal/Urinary Metanephros at level of sacrum
  Renal/Urinary Urethral plate
  Gastrointestinal Lesser omentum (ventral mesogastrun)
  Gastrointestinal Peritoneal cavity
  Gastrointestinal Rectum
  Gastrointestinal Stomach: Greater and lesser curvatures
  Gastrointestinal Yolk stalk disappears
  Respiratory Bronchial tree expanding
  Respiratory Cervical sinus diminished in size
  Respiratory Epiglottis
  Respiratory Primitive Larynx
  Cardiovascular Anterior, middle, and posterior cerebral arteries
  Cardiovascular Atrioventricular (AV) node
  Cardiovascular Atrioventricular cushions fuse with interventricular septum
  Cardiovascular Circle of Willis almost complete
  Cardiovascular Conotruncal septum
  Cardiovascular Endocardial cushions (rostroventral and caudodorsal) begin fusing around atrioventricular canal forming right and left atrioventricular canals and two separate blood streams
  Cardiovascular External carotid artery
  Cardiovascular Foramen primum disappearing
  Cardiovascular Hepatic portal vein
  Cardiovascular Infundibulum of right ventricle
  Cardiovascular Jugular lymph sac
  Cardiovascular Lateral atrioventricular cushions
  Cardiovascular Mesencephalic artery
  Cardiovascular Myelencephalic artery
  Cardiovascular Perilental blood vessels
  Cardiovascular Primitive cavernous sinus drains primitive maxillary and supraorbital veins
  Cardiovascular Primitive renal plexus
  Cardiovascular Right ventricle feeds sixth (pulmonary) aortic arches; left ventricle feeds fourth aortic arches
  Cardiovascular Semilunar valves (aortic and pulmonary) are forming
  Cardiovascular Ventricles each with three parts: inlet, trabecular pouch, and outflow tract
  Cardiovascular Ventricles enlarge and deepen side-by-side forming an ever growing interventricular septum
  Nervous Celiac plexus
  Nervous Cochlear nerve present
  Nervous Femoral and obturator nerves innervate rostrolateral part of lower limb
  Nervous Hypoglossal nerve (CN XII) reaches tongue
  Nervous Intercostal nerves
  Nervous Lumbar and sacral plexuses
  Nervous Musculocutaneous, radial, ulna, and median nerves enter upper limb bud
  Nervous Nasal pits face more ventrally, still widely separated
  Nervous Nasofrontal groove
  Nervous Olfactory fibers connect nasal pits with brain
  Nervous Olfactory fibers enter brain
  Nervous Olfactory tubercle present
  Nervous Peroneal and tibial nerves innervate caudomedial part of lower limb
  Nervous Phrenic nerve
  Nervous Pigment in retina visible externally
  Nervous Primordium of cochlear pouch
  Nervous Tibial nerve innervates foot area
  Nervous Auricular hillocks on hyoid arch (antitragus and helix)
  Nervous Auricular hillocks on mandibular arch (tragus and crus)
  Nervous Blind nasal sac
  Nervous Nasal fin
  Special Senses Alar lamina emerging with dense rhombic lip
  Special Senses All cranial nerves identifiable
  Special Senses Archipallium, paleopallium, and neopallium
  Special Senses Area epithelialis
  Special Senses Brain: Primordial plexiform layer in area of future temporal lobe
  Special Senses Cajal-Retzius cells
  Special Senses Commissure of the trochlear nerve
  Special Senses Diencephalic subthalamic nucleus
  Special Senses Dorsal and ventral thalami separated by groove
  Special Senses Dorsal funiculus fibers reach medulla oblongata
  Special Senses Epiphysis cerebri
  Special Senses Glial cells identifiable adjacent to neurons
  Special Senses Greater petrosal nerve
  Special Senses Hippocampus: Gyrus dentatus
  Special Senses Infundibular recess and infundibulum
  Special Senses Interventricular foramen large
  Special Senses Marginal ridge
  Special Senses Medial and lateral ridges of corpus striatum are continuous
  Special Senses Median forebrain bundle
  Special Senses Neurohypophysial outgrowth
  Special Senses Olfactory tubercle
  Special Senses Pontine flexure deepens
  Special Senses Posterior commissure
  Special Senses Recurrent laryngeal nerve
  Special Senses Reticular formation more defined
  Special Senses Retinal fissure closes
  Special Senses Splanchnic nerve
  Special Senses Sulcus limitans hippocampi
  Special Senses Superior laryngeal nerve
  General Second pharyngeal arch more prominent
  General Third pharyngeal arch recedes
  Skeletal Initial tooth formation
  Movement Subtle movement begins
5 weeks, 4 daysSkeletal Cartilage formation
5 weeks, 5 daysSpecial Senses Nerve cells differentiating
5 weeks, 5 days - 7 weeks, 1 daySkin Melanocytes in epidermis
5 weeks, 6 daysSkin Facial growth centers grow and begin merging forming nose and upper jaw
  Reproductive Genital eminence forms phallus or genital tubercle
  Reproductive Gonad grows into oval shape with irregular surface
  Skeletal Auditory ossicles identifiable in mesenchyme
  Skeletal Cartilage in occipital sclerotomes (1-4)
  Skeletal Digital rays in hand plate
  Skeletal Femur: Chondrification begins
  Skeletal Foot with rounded digital plate
  Skeletal Hypoglossal foramen (or canal) through sclerotome 4 (area of future occipital bone)
  Skeletal Odontogenic epithelium ermerges in six areas (four maxillary and two mandibular)
  Skeletal Primary palate components (right and left) fuse in midline
  Skeletal Primitive palatine groove
  Skeletal Primordium of cartilage within nasal septum
  Skeletal Vertebral centra begin chondrification
  Muscular Primordia of orbital muscles
  Renal/Urinary Calices
  Renal/Urinary Mesonephros can produce urine
  Renal/Urinary Pelvis of the ureter with three main divisions
  Renal/Urinary Vesico-urethral canal
  Gastrointestinal Biliary ducts within liver
  Gastrointestinal Dorsal and ventral pancreas fuse but retain separate ducts
  Gastrointestinal Duodenum enlarges proximal to and distal to bile and pancreatic ducts
  Gastrointestinal Esophagus developing a submucous coat surrounding epithelium
  Gastrointestinal Intestinal loop begins umbilical herniation
  Gastrointestinal Primordial vermiform appendix
  Gastrointestinal Stomach regions include gastric canal, fundus, corpus (or body), and pyloric antrum
  Respiratory Trachea: Precursors of tracheal cartilages
  Cardiovascular Anterior choroid artery
  Cardiovascular Condensing mesenchyme around junction between left and right atria and cardiac tube is precursor to mitral and tricuspid valves
  Cardiovascular Outflow tract rotates counterclockwise
  Cardiovascular Right and left atrioventricular canals totally separated
  Nervous All parasympathetic cranial nerve ganglia identifiable
  Nervous All spinal nerves present
  Nervous Cell islands in olfactory tubercle
  Nervous Crescentic lens cavity
  Nervous Geniculate ganglion separate from vestibulocochlear nerve
  Nervous Globular process emerges from each medial nasal process
  Nervous Nasal fin connecting nasal disc and surface epithelium
  Nervous Nasofrontal grooves
  Nervous Olfactory tubercle with cellular islands
  Nervous Hyomandibular groove enlarges (onset of concha and external auditory meatus formation)
  Nervous Medial rims of nasal pits form nasal septum
  Nervous Nostril becomes continuous with nasal sac
  Nervous Primary lens fibers
  Nervous Retinal fissure closed
  Special Senses Capillaries between adenohypophysis and hypothalamus
  Special Senses Commissure of the oculomotor nerves
  Special Senses Cortical nucleus in amygdaloid body
  Special Senses Dentate and isthmic nuclei in cerebellum
  Special Senses Dura begins forming in basal area
  Special Senses Epiphysis cerebri with intermediate layer
  Special Senses First hint of septal nucleus
  Special Senses Frontal and temporal poles of cerebral hemispheres
  Special Senses Gustatory fibers separate from common afferent tract
  Special Senses Hemispheric stalk
  Special Senses Intermediate layer in tectum mesencephali
  Special Senses Interventricular foramen
  Special Senses Mesencephalon with intermediate layer
  Special Senses Somites: Pairs 38 and 39
  Special Senses Spinal cord reaches caudal tip of body
  Special Senses Subarachnoid space
  Special Senses Synapses in spinal cord between interneurons and primary afferent neurons
  Special Senses Ventral thalamus with intermediate layer
6 weeksMovement Face withdraws from light touch around mouth
  Blood & Immune Blood forming in liver
  Skin Milk lines
  Skin Nipples along side of trunk
  Endocrine Adrenal glands
  Endocrine Glucagon in pancreas
  Skeletal Handplates develop subtle flattening
  Skeletal Joints
  Skeletal Medial skull cartilages: Parachordal, hypophyseal, and trabecular
  Skeletal Tooth buds (primary teeth)
  Muscular Diaphragm is largely formed
  Gastrointestinal Intestines fill base of umbilical cord
  Gastrointestinal External ears
  Special Senses Synapses form in spinal cord
  General Crown-heel length 1.6 cm
Unit 7:   6 to 7 Weeks 
6 weeks, 2 daysReproductive Angiogenesis begins inside gonads
  Reproductive Gonad grows into oval shape with irregular surface
  Reproductive Ostium (abdominal) of uterine tube at rostral end of paramesonephric duct (in female embryos)
  Reproductive Paramesonephric duct forms from rostral end of mesonephric duct
  Reproductive Testicular cords in gonads of male embryos
  Reproductive Testicular cords in male gonad
  Skeletal Elbow regions sometimes identifiable
  Skeletal Embryo with cervical and lumbar flexures
  Skeletal Embryo with dorsal concavity
  Skeletal Finger rays with early interdigital notching
  Skeletal Hands polygon-shaped
  Skeletal Humerus, radius, and ulna
  Skeletal Humerus: Chondrocytes in phases one through three
  Skeletal Scapula and clavicle
  Skeletal Semicircular ducts form in order: anterior, posterior, and lateral
  Skeletal Sternum: Episternal cartilage created from fusion of right and left sternal bars
  Skeletal Tibia and fibula
  Skeletal Toe rays sometimes present
  Muscular Deltoid muscle
  Muscular External and internal abdominal oblique muscles
  Muscular Levator scapulae muscle
  Muscular Longus cervicis and semispinalis cervicis muscles
  Muscular Pectoralis major muscles
  Muscular Platysma muscle
  Muscular Rectus abdominis muscle
  Muscular Rectus capitus posterior and semispinalis capitis muscles
  Muscular Serratus anterior muscles
  Muscular Splenius and longissimus muscles
  Muscular Stapedius muscle
  Renal/Urinary "Common excretory duct is disappearing"
  Renal/Urinary Cloacal membrane ruptures (stages 18-19)
  Renal/Urinary Primordia of secretory tubules
  Gastrointestinal Esophagus with muscular and submucous coats
  Gastrointestinal Submandibular gland primordia
  Respiratory Bronchial tree with subsegmental buds
  Respiratory Bronchial tree with well established segmental bronchi
  Respiratory Lingula of left upper lobe
  Cardiovascular Aortic and pulmonary valves assuming shape of a cup
  Cardiovascular Brachiocephalic veins, right and left
  Cardiovascular Inferior vena cava
  Cardiovascular Interventricular septum: membranous part begins forming
  Cardiovascular Left subclavian artery feeds left axillary and vertebral arteries and left thyrocervical trunk
  Cardiovascular Mesenchyme ridges in place of future mitral and tricuspid valves
  Cardiovascular Origin of left coronary artery
  Cardiovascular Pulmonary and aortic blood flows completely separate
  Cardiovascular Right subclavian artery originates from brachiocephalic artery and feeds right thyrocervical trunk and axillary and vertebral arteries
  Cardiovascular Secondary interventricular foramen sometimes closing (stage 18-21) interventricular septum
  Cardiovascular Septum secundum and foramen ovale (stages 18-21)
  Nervous Bucconasal membrane
  Nervous Bucconasal membrane detaches opening up nasal airway
  Nervous Crus commune
  Nervous Ethmoidal epithelium emerges from upper medial nasal wall
  Nervous Frontonasal angle (marks location of future nasal bridge)
  Nervous Mesenchyme thickenings mark beginning of "sclera and its muscular attachments"
  Nervous Nasal tip emerges
  Nervous Nerve fibers in retina
  Nervous Optic fibers
  Nervous Retina's outer lamina heavily pigmented
  Nervous Vomeronasal nerve and ganglion
  Nervous Vomeronasal organ marked by groove and located in fold of lower medial nasal wall
  Nervous Choanae
  Nervous Conjunctival sac marked by groove
  Nervous Cornea and conjunctiva
  Nervous Ear: Stapes primordium surrounds stapedial artery
  Nervous External ear: Crus helicis forming from auricular hillocks two and three (from mandibular arch)
  Nervous Eyelid folds sometimes present
  Nervous Nasal fin splits forming choanae and bucconasal membrane
  Nervous Nasolacrimal duct begins as epithelial strand emanating from nasomaxillary groove
  Nervous Nostrils, nasal wings, and nasal septum easily seen
  Nervous Olfactory bulb sometimes with olfactory ventricle
  Nervous Primary lens fibers filling lens vesicle cavity
  Special Senses Adenohypophysis no longer open to pharyngeal cavity
  Special Senses Archistriatum
  Special Senses Brain: Dentate nucleus in internal cerebellar swellings
  Special Senses Brain: Pineal recess emerges representing anterior lobe of epiphysis
  Special Senses Brainwave activity has begun
  Special Senses Cerebrospinal fluid production begins
  Special Senses Choroid plexuses in fourth and lateral ventricles
  Special Senses Corpus striatum much larger extending to preoptic sulcus; has subtle groove
  Special Senses External cerebellar swellings contain future flocculus
  Special Senses Four amygdaloid nuclei
  Special Senses Fourth ventricle: Choroid folds
  Special Senses Hippocampus reaches olfactory region
  Special Senses Interpeduncular fossa
  Special Senses Neurohypophysis walls are folded
  Special Senses Nucleus ambiguus of the vagus (CN10)
  Special Senses Prosencephalic septum
  Special Senses Red nucleus
  Special Senses Substantia nigra
  Special Senses Supraoptic commissure
  Movement The hands begin to move
  Skin Volar pads on palms
  Skeletal Bones first form in the collar bones and lower jaw
6 weeks, 5 daysBlood & Immune Greater thymic bud
  Skin Cheeks form by merging of maxillary and mandibular processes
  Skin Mammary gland primordium
  Skin Mammary ridge disappears leaving only mammary gland primordium
  Reproductive Female duct
  Reproductive Gonads extend from levels T-10 to L-2
  Reproductive Rete ovarii (in female embryos)
  Reproductive Rete testis begins emerging from seminiferous cords (Stage 19-23) (in male embryos)
  Reproductive Tunica albuginea in male embryos
  Endocrine Suprarenal gland: Cortex
  Endocrine Suprarenal gland: Medulla populated by prechromaffin cells
  Skeletal Arms point forward
  Skeletal Beginnings of occipital and sphenoid bones
  Skeletal Bilateral cartilaginous sternal bars tie ribs together; sternal bars join cranially to form the episternal bar in the midline
  Skeletal Cartilage within otic capsule envelops semicircular canals and cochlear duct
  Skeletal Cartilaginous styloid process
  Skeletal Ear: Cartilaginous malleus, incus, and stapes (the middle ear ossicles)
  Skeletal Ectomeninx covers lateral and dorsal surfaces of brain (laying the foundation for the flat bones of the skull)
  Skeletal Intervertebral discs form from caudal condensed portion of sclerotomes
  Skeletal Ischium and illium
  Skeletal Labiodental lamina: Inner dental lamina and outer labiogingival band
  Skeletal Laryngeal cartilages
  Skeletal Limbs point forward (ventrally)
  Skeletal Orbitosphenoid cartilage located within ectomeninx near optic stalk
  Skeletal Ossification begins in maxilla (stages 19 -20)
  Skeletal Primitive palate (or intermaxillary segment)
  Skeletal Rib primordia become cartilaginous
  Skeletal Ribs each have an identifiable head and shaft
  Skeletal Trachea: Tracheal cartilage
  Skeletal U-shaped labiodental lamina form along upper and lower oral cavity
  Skeletal Vertebral column represented by cartilaginous centrum, neural arch, and short tranverse process
  Muscular Esophagus: Muscularis layer adjacent to esophageal plexus
  Muscular Gluteal muscle group
  Muscular Iliopsoas muscles
  Muscular Infrahyoid muscles
  Muscular Internal intercostal muscles
  Muscular Limb extensor muscles located dorsally
  Muscular Limb flexor muscles located ventrally
  Muscular Midgut: Muscularis
  Muscular Muscle tissue forming around phrenic nerve within septum transversum portion of diaphragm
  Muscular Pharyngeal constrictor muscle
  Muscular Premuscle mass of the muscles of mastication innervated by mandibular nerve
  Muscular Quadratus lumborum muscle
  Muscular Rhomboid and scalene muscles
  Muscular Sternocleidomastoid and trapezius muscles distinct and innervated by separate branches of spinal accessory nerve (CN XI)
  Muscular Thenar and hypothenar eminences
  Muscular Tongue forms from swellings in floor of pharynx
  Muscular Tongue: Extrinsic muscles identifiable
  Muscular Tongue: Intrinsic muscles identifiable
  Muscular Transversospinal and erector spinae muscle groups
  Muscular Upper limb flexors innervated by musculocutaneous, median, and ulnar nerves
  Renal/Urinary Major calyces, cranial and caudal, with collecting tubules within metanephrogenic mass
  Renal/Urinary Mesonephros extends from T-9 to L-3
  Renal/Urinary Metanephros extends from T-12 to L-2
  Renal/Urinary Renal capsule covers distal collecting tubules
  Renal/Urinary Renal vesicles form in part of metanephros
  Renal/Urinary Ureter forms from "proximal segment of metanephric diverticulum"
  Renal/Urinary Urogenital sinus comprised of three parts: Bladder, pelvic, and phallic portions
  Gastrointestinal Anal folds adjacent to anal membrane
  Gastrointestinal Anal membrane
  Gastrointestinal Duodenum: "Assumes the shape of an arc"
  Gastrointestinal Greater omentum
  Gastrointestinal Lateral palatine process
  Gastrointestinal Liver: rapid growth, right side greater than left
  Gastrointestinal Median mandibular groove disappears as mandibular processes merge in midline
  Gastrointestinal Palatine fossa (from pharyngeal pouch 2)
  Gastrointestinal Primitive oral cavity
  Gastrointestinal Primitive rima oris replaces stomodeum
  Gastrointestinal Stomach wall layers: Mucosa, submucosa, muscularis, and serosa
  Gastrointestinal Submandibular and parotid gland buds
  Gastrointestinal Submandibular gland duct
  Respiratory Bronchial tree: First generation of subsegmental bronchi complete
  Respiratory Glottis, primitive
  Respiratory Lung sac, right: Oblique and horizontal fissures define upper, lower, and middle lobes
  Respiratory Lung sac: Apex and base
  Respiratory Lung, left: Oblique fissure defines upper and lower lobes
  Cardiovascular "Septum primum fuses with endocardial cushions" obliterating ostium primum and creating the ostium secundum
  Cardiovascular Apex of left ventricle
  Cardiovascular Circulus arteriosus (Circle of Willis) complete
  Cardiovascular External iliac arteries
  Cardiovascular Iliac lymph sac
  Cardiovascular Intercostal and subcostal arteries
  Cardiovascular Internal thoracic artery and costocervical trunk
  Cardiovascular Mesenteric lymph sac
  Cardiovascular Mesonephric artery feeds mesonephros, gonads, and suprarenal glands
  Cardiovascular Origin of right coronary artery
  Cardiovascular Paired common iliac veins feed inferior vena cava
  Cardiovascular Papillary muscles
  Cardiovascular Pontine, superior cerebellar, and anterior and posterior interior cerebellar arteries replace myelencephalic and metencephalic arteries
  Cardiovascular Primitive marginal sinus drains diencephalon
  Cardiovascular Primitive tentorial sinus drains cerebral vesical
  Cardiovascular Primitive transverse and sigmoid sinuses
  Cardiovascular Pulmonary arteries (right and left)
  Cardiovascular Splenic vein
  Cardiovascular Tricuspid and mitral valves
  Nervous Anterior chamber between iridopupillary membrane and thickened ectoderm
  Nervous Auditory tube and primtive tympanic cavity form from tubotympanic recess pharyngeal pouch 1)
  Nervous Celiac, superior mesenteric, and inferior mesenteric preaortic ganglia
  Nervous Choana
  Nervous Cochlear duct tip grows upward
  Nervous Esophageal plexus formed by vagal nerves (CN X)
  Nervous Facial nerve (CN VII) branches: Chorda tympani, greater petrosal, posterior auricular, and digastric
  Nervous Facial nerve (CN VII) reaches cervicomandibular region
  Nervous Glossopharyngeal nerve (CN IX) innervates stylopharyngeus premuscle mass
  Nervous Hypoglossal nerve (CN XII) innervates separating tongue muscles
  Nervous Linguogingival groove
  Nervous Nasolacrimal duct forms from maxillonasal groove
  Nervous Nasolacrimal ducts extend from medial eyes to primitive nasal cavity
  Nervous Nerve fibers begin extending from retina
  Nervous Optic fibers enter chiasmatic plate
  Nervous Primitive nasal cavity
  Nervous Primordial vitreous body
  Nervous Superior, middle, and inferior cervical ganglia
  Nervous Trigeminal nerve (CN V) with opthalmic, maxillary, and mandibular divisions reach their destinations
  Nervous Vagal trunks, anterior and posterior, extending into abdomen
  Nervous Eyelids: Upper and lower lids present and growing
  Nervous Saccule and cochlear duct
  Special Senses Adenohypophysis: Lateral lobes of pars tuberalis
  Special Senses Adenohypophysis: Pars intermedia emerging
  Special Senses Brain: Internal capsule formation underway
  Special Senses Cerebral hemispheres cover half of diencephalon
  Special Senses Dorsal and ventral cochlear nuclei
  Special Senses Fourth ventricle: Lateral recesses
  Special Senses Ganglion of nervus terminalis
  Special Senses Globus pallidus externus in the diencephalon
  Special Senses Habenular commissure
  Special Senses Intermediate layer in dorsal thalamus
  Special Senses Lemniscal decussation
  Special Senses Lower limb nerves (femoral, obturator, sciatic, common peroneal, and tibial) identifiable
  Special Senses Medial accessory olivary nucleus
  Special Senses Neurohypophyseal bud
  Special Senses Nuclei of forebrain septum
  Special Senses Nucleus accumbens
  Special Senses Occipital pole of cerebral hemispheres
  Special Senses Optic stalk with barely discernible lumen
  Special Senses Paraphysis marks dividing line in roof between telencephalon and diencephalon
  Special Senses Primitive filum terminale
  Special Senses Radial nerve innervates upper limb extensors
  Special Senses Rhombomeres no longer distinguishable
  Special Senses Subcommissural organ
  Special Senses Zona limitans intrathalamica between dorsal and ventral thalami
6 weeks, 6 daysSkeletal Feet polygon-shaped
  Gastrointestinal Cloacal membrane ruptures
7 weeksMovement Head rotates
  Movement Leg movements
  Blood & Immune B lymphocytes in liver
  Reproductive Ovaries
  Reproductive Testes begin to differentiate
  Endocrine Insulin in pancreas
  Skeletal Foot plates notched
  Muscular Hiccups
  Muscular Tendons attach muscle to bone
  Cardiovascular The heart has four chambers and is nearly complete.
  Cardiovascular The heart rate peaks at 165 to 170 beats per minute.
  General Crown-heel length 2.2 cm
Unit 8:   7 to 8 Weeks 
7 weeks, 1 daySkin Facial processes no longer distinguishable
  Reproductive Ovaries full of primitive oogonia, intermediate pregranulosa cells, and mesenchyme
  Reproductive Testes with short straight tubules
  Skeletal Upper limbs with slightly flexed elbows
  Muscular Diaphragm: Central tendon
  Renal/Urinary Renal vesicles with S-shaped lumina
  Gastrointestinal Submandibular gland: Solid epithelial ducts enlarge and begin to branch
  Cardiovascular Adenohypophysis with new capillaries on rostral surface
  Cardiovascular Scalp vascular plexus
  Nervous Cochlear duct tip growing horizontally
  Nervous Lens cavity completely filled
  Nervous Optic commissure
  Nervous Optic fibers extend to optic chiasma
  Nervous Cornea with three layers
  Special Senses Brain: Inferior colliculus (in mesencephalon)
  Special Senses Cerebral hemispheres expand beyond lamina terminalis
  Special Senses Cerebral hemispheres extend over two-thirds of diencephalon
  Special Senses Interpeduncular groove
  Special Senses Medial septal nucleus
  Special Senses Nigrostriatal fibers
  Special Senses Nucleus of diagonal band
  Special Senses Sacrocaudal spinal cord formation (secondary neurulation) complete
  Special Senses Sensory pathways: Cuneate and gracile decussating fibers
  Special Senses Septum verum
  Special Senses Spinothalamic tract
7 weeks, 1 day - 8 weeksGastrointestinal Stomach: Folds in stomach wall
7 weeks, 2 daysCardiovascular Arteries and veins of heart complete
7 weeks, 3 daysSkin Volar pads begin to emerge on fingertips
  Skeletal Chondrocranium with dorsum sellae and hypophysial fossa
  Skeletal Dens (of second cervical vertebrae)
  Skeletal Sternoclavicular joint and manubrium
  Skeletal The knee joints have arrived
  Skeletal Trachea: Thyroid cartilage
  Skeletal Wrists slightly flexed
  Muscular Gluteus medius and gluteus minimus muscles
  Muscular Iliacus muscles
  Muscular Mylohyoid and infrahyoid muscles
  Muscular Orbicularis oculi muscles
  Gastrointestinal Submandibular gland: Solid ducts with definitive branches
  Cardiovascular Anterior and posterior choroid arteries
  Cardiovascular Left superior vena cava disappears (Stages 21-23)
  Cardiovascular Scalp vascular plexus moving toward vertex
  Nervous Cornea: Substantia propria layer
  Nervous Fibers of optic nerve reach brain
  Nervous Eyelids growing rapidly
  Special Senses Anterior and inferior horns of lateral ventricle
  Special Senses Brain: Insula within cerebral hemisphere
  Special Senses C-shaped lateral ventricle
  Special Senses Cerebral hemispheres cover 75% of diencephalon
  Special Senses Cerebral hemispheres cover more than half of diencephalon
  Special Senses Cortical plate within primordial plexiform layer
  Special Senses Glial and neurilemmal (Schwann) cells within cranial nerves
  Special Senses Global pallidus internus
  Special Senses Internal fiber layer of cerebellum
  Special Senses Lateral olfactory tract
  Special Senses Primordium of dentate nucleus
  Special Senses Pyramidal cells in hippocampus
  Special Senses Subthalamic nucleus proper, entopeduncular nucleus, and globus pallidus externus within subthalamus
  Special Senses Sulcus transversus rhombencephali
  Special Senses Ventral part of lateral geniculate body
  Movement Hands begin to touch face
  Movement The hands touch each other as do the feet!
  Skin Fingertips thicken
  Skin Plantar pads toes
  Cardiovascular EKG pattern similar to adult
7 weeks, 4 daysSkeletal The fingers are free
7 weeks, 5 daysSkeletal Bone-forming cells called osteoblasts emerge
  Skeletal Bone-forming cells emerge
  Skeletal Endolymphatic and jugular foramina
  Skeletal Hands can reach one another and fingers can overlap
  Skeletal Optic foramen, foramen rotundum, internal acoustic foramen
  Skeletal Osteoblasts emerge
  Skeletal Pelvis: Obturator foramen
  Muscular Obturator internus muscles
  Muscular Rectus femoris muscle
  Renal/Urinary Large glomeruli present within metanephros
  Gastrointestinal Submandibular gland: Secondary branching with lumen formation starting at oral end of duct
  Respiratory Costodiaphragmatic recess of pleural cavity
  Cardiovascular Chordae tendineae (Stages 22 and 23)
  Cardiovascular Intradural veins (sinuses)
  Cardiovascular Scalp vascular plexus 75% of the way to the vertex
  Nervous Cochlear duct's second loop growing upward
  Nervous Scleral condensation
  Nervous Tragus and antitragus taking shape
  Nervous Eyelids continue growing rapidly over the surface of the cornea
  Nervous Optic nerve acquires a sheath
  Special Senses Brain: Claustrum
  Special Senses Brain: Cortical plate within cerebral hemispheres
  Special Senses Brain: Internal capsule with connections to epithalamus, dorsal thalamus, and mesencephalon
  Special Senses Brain: Putamen
  Special Senses Cerebral hemispheres cover 75% of diencephalon
  Special Senses Commissural plate thickens
  Special Senses Cortical plate expanding rapidly
  Special Senses Folds in roof of third ventricle
  Special Senses Nerve fibers between neopallial subplate and internal capsule
  Special Senses Thalamocortical fibers
7 weeks, 6 daysSkeletal The toes are free
8 weeksMovement Complex response to touch
  Movement More frequent hand-to-face contact
  Movement Mouth opens & closes
  Movement Squinting
  Movement The embryo floats and rolls over in the womb
  Skin Hairs first appear in eyebrows & around mouth
  Skin Skin multi-layered, loses transparency
  Reproductive Ductus deferens
  Reproductive Interstitial cells forming within testis
  Reproductive Testicular tubules
  Endocrine Male embryos are making testosterone already!
  Skeletal Anterior inferior iliac spine
  Skeletal Costal cartilage
  Skeletal Enamel organ
  Skeletal Femur: Head and acetabular fossa
  Skeletal Glenoid fossa
  Skeletal Greater trochanter
  Skeletal Head of humerus
  Skeletal Inguinal ligament
  Skeletal Joint development: Cavitation underway in hip, knee, and ankle (in some embryos)
  Skeletal Joint development: Cavitation underway in shoulder, elbow, and wrist (in some embryos)
  Skeletal Nucleus pulposus (from notochord)
  Skeletal Ossification underway in scapula and distal phalanges in some embryos
  Skeletal Pubic symphysis
  Skeletal Scapular spine and notch
  Skeletal Skull: Foramen magnum (wide)
  Skeletal Skull: Ossification underway in some embryos
  Skeletal Superior and inferior pubic rami
  Skeletal The embryo's joints are similar to adult joints
  Skeletal Ulna: Styloid process and olecranon
  Skeletal Vertebrae cartilaginous (33 or 34 in number)
  Muscular Anterior digastric muscles
  Muscular Depressor anguli oris muscle
  Muscular Diaphragm complete
  Muscular Esophagus: Longitudinal muscles
  Muscular Obliquus superior capitus muscle
  Muscular Obturator externus, gluteus maximus, and hamstring muscles
  Muscular Posterior belly of the digastric muscle
  Muscular Psoas tendon
  Muscular Rectus sheath with anterior and posterior lamina
  Muscular Temporal and lateral pterygoid muscles
  Muscular Zygomaticus major muscle
  Renal/Urinary Bladder: Muscularis layer
  Renal/Urinary Kidneys at level of first three lumbar vertebrae
  Renal/Urinary Metanephros: Numerous large glomeruli
  Renal/Urinary Metanephros: Secretory tubules elongating and becoming convoluted
  Renal/Urinary Sinusal tubercle
  Renal/Urinary Urethra
  Renal/Urinary Urine production and release
  Gastrointestinal Gastrolienal ligament
  Gastrointestinal Nerves reaching intestinal loop
  Gastrointestinal Peristalsis in large intestine
  Gastrointestinal Submandibular gland: Lumen present in terminal portions of duct
  Gastrointestinal Submandibular gland: Mesodermal sheath surrounds gland
  Gastrointestinal Unfused uvula (edge of unfused palatine shelf) and secondary palate
  Respiratory Occasional breathing motions begin
  Respiratory Pseudoglandular stage begins
  Cardiovascular Azygos vein
  Cardiovascular Blood supply to the brain closely resembles adult pattern
  Cardiovascular Hemiazygos veins
  Cardiovascular Inferior epigastric artery
  Cardiovascular Inferior vena cava valve at junction of right atrium
  Cardiovascular Scalp vascular plexus nearing vertex
  Cardiovascular Submandibular glands: Angiogenesis begins around epithelial tree (ducts)
  Cardiovascular Superior sagittal sinus
  Nervous Cochlear duct's 2.5 coils nearly complete
  Nervous Cranial nerve distribution mimics adult pattern
  Nervous Ear drum
  Nervous Eye: Secondary vitrous body
  Nervous Lens: Secondary lens fibers emerging
  Nervous Retina: Eight layers present
  Nervous Retina: Four of the ten adult layers present
  Nervous Tympanic membrane
  Nervous Eyelids fusing laterally and medially
  Nervous Optic tract reaches ventral portion of lateral geniculate body
  Special Senses "The hindbrain "presents striking resemblance to that of the newborn."
  Special Senses "The rhombencephalon...presents striking resemblance to that of the newborn."
  Special Senses Amygdala area
  Special Senses Brain represents 43% of embryo
  Special Senses Brain: Caudate nucleus and putamen within corpus striatum
  Special Senses Cerebellar commissures
  Special Senses Cerebellum with external germinal layer
  Special Senses Cerebral hemispheres cover lateral portion of diencephalon
  Special Senses Choroid plexus now lobular
  Special Senses Cortical plate covers nearly all of neopallial surface
  Special Senses Dura lines entire vertebral canal
  Special Senses Fasciculus cuneatus and fasciculus gracilis form the decussation of the medial lemnisci
  Special Senses Greater palatine nerve
  Special Senses Grey and white matter
  Special Senses Hippocampus reaches temporal pole
  Special Senses Inferior and superior cerebellar peduncles
  Special Senses Most cisterns present
  Special Senses Principal nucleus of inferior olivary nuclei
  Special Senses Pyramidal decussations
  Special Senses Right- and left-handedness emerges
  Special Senses Suprapineal recess
  Special Senses Suprascapular nerve
  Special Senses Vermis of cerebellum
  General Crown-heel length 4.3 cm
  General Embryo contains approximately 1 billion (10^9) cells
  General Embryonic Period Ends
  General The embryo has more than 4,000 parts (adults have about 4,500)
Unit 9:   8 to 9 Weeks 
8 weeks, 1 dayBlood & Immune Humerus: Bone marrow replaces cartilage
8 weeks, 1 day - 9 weeksGastrointestinal Anal canal patent
  Gastrointestinal Eyelids completely fused
  Special Senses Neurons synapse in cerebral cortex (marginal zone)
9 weeksMovement Bends hip & knee if sole of foot touched
  Movement Drinking fluid is becoming routine
  Movement Sucking the thumb
  Movement The young fetus now sighs, stretches, moves the head, opens the mouth, and moves the tongue
  Movement Tongue movement
  Reproductive Female fetuses have early reproductive cells in their ovaries
  Endocrine Thyroid gland weighs 2 grams
  Gastrointestinal Small intestine peristalsis
  Special Senses External capsule
  Special Senses Face, hands, and feet sense light touch
  Special Senses Olivary nucleus with five components
Unit 10:   9 to 10 Weeks 
9 weeks - 10 weeksRespiratory Early vocal cords
  Respiratory Larynx recanalizes
  General My weight will rise more than 75% this week
  Movement I yawn when I want
9 weeks, 4 daysSpecial Senses Yawns
10 weeksMovement Eyes roll downward reflexively
  Blood & Immune Palatine tonsils
  Skin Fingernails and toenails begin to grow!
  Skin Three-layered epidermis
  Skin Tiny unique fingerprints have arrived!
  Skeletal Now, all the bones are getting harder
  Skeletal Tooth buds (secondary teeth)
  Renal/Urinary Glomeruli formation begins
  Gastrointestinal Physiologic herniation ends
  Special Senses Commissure of the fornix
  Special Senses Corpus callosum begins
  General Crown-heel length 7.5 cm
Unit 11:   10 to 11 Weeks 
10 weeks - 12 weeksBlood & Immune Langerhans cells enter epidermis
  Skin Volar and plantar pads regress
11 weeksMovement The face now makes complex expressions
  Blood & Immune Immunological competence
  Skin Intermediate layer
  Skin Nose & lips completely formed
  Reproductive Now you can tell if your baby is a girl or a boy!
  Endocrine Thyroid gland weighs 12 grams
  Gastrointestinal Intestines absorb water & glucose
  Gastrointestinal Small intestine lined with villi
  Nervous Auditory cells: inner & outer hair cells
  General Crown-heel length
Unit 12:   11 to 12 Weeks 
11 weeks - 12 weeksGeneral Weight increases by 60% this week
12 weeksMovement Hands touch the mouth up to 50 times per hour
  Blood & Immune T lymphocytes leave thymus
  Skin Sebaceous glands
  Endocrine Many different hormones are present in pituitary gland
  Endocrine Thyroid gland produces hormone
  Skeletal Palate fuses
  Skeletal Upper limbs reach final proportion
  Muscular All facial muscles in final positions
  Renal/Urinary Bladder resembles smooth muscle
  Gastrointestinal Bile production begins in liver
  Gastrointestinal Bowel movements
  Nervous There are taste buds all over the mouth
  Special Senses Corpus callosum
  Special Senses Crura cerebri
  Special Senses Myelination in spinal cord
  General Crown-heel length 12 cm
  General Head circumference 10 cm
Unit 13:   3 to 4 Months 
13 weeksSkeletal Teeth are growing
  Respiratory Cilia lining airways
  Special Senses Most of body sensitive to touch
  General Crown-heel length 15 cm
14 weeksMovement Girls move their jaws more than the boys do
  Movement Light touch to mouth evokes turn toward stimulus
  Special Senses 4-lobed cerebral cortex
  Special Senses Cerebellum resembles adult structure
  General Crown-heel length 17 cm
  General Fat deposits in cheeks
15 weeksBlood & Immune Stem cells arrive in bone marrow
  Skin Body fat emerges throughout the body
  Endocrine Glucagon in fetal bloodstream
  Gastrointestinal Digestive enzymes
  General Crown-heel length 19.5 cm
16 weeksMovement Quickening
  Skin Fat deposits upper & lower limbs
  Skeletal Tooth enamel
  Gastrointestinal Colon lined with villi
  Respiratory Bronchial tree nearly complete
  Respiratory Canalicular stage begins
  Nervous Hormonal stress response to invasive procedures
  General Crown-heel length 21 cm
Unit 14:   4 to 5 Months 
17 weeksNervous Retina has discrete layers
18 weeksSkin Apocrine sweat glands
  Skin Cream-like substance protects skin
  Skin Sweat glands
  Endocrine Insulin secretion
  Respiratory Speaking motion of larynx
  Special Senses Corpus callosum complete
19 weeksSkin Melanin production
  Reproductive Number of oogonia peak (at about 7 million) within fetal ovaries
  Special Senses Daily cycles in biological rhythms
  Special Senses Sulci on surface of cerebral hemispheres
20 weeksSkin All skin layers and structures
  Gastrointestinal Peyer's patches
  Respiratory Surfactant production (low levels)
  Nervous Hearing and responding to sound begins
  Nervous Hearing and responding to sound begins
  General Crown-heel length 28 cm
  General Head circumference 20 cm
Unit 15:   5 to 6 Months 
20 weeks - 24 weeksGeneral Eyelids separate, eyes open and close
21 weeksSkin Periderm disappears
  Skin Stratum corneum
21 weeks - 22 weeksGeneral If born prematurely from this point on, survival is possible
22 weeksGeneral Cornea structure
  Special Senses Behavioral states
23 weeksGeneral Brain weight 100 grams
24 weeksNervous Blink-startle response; females before males
  General Crown-heel length 34.5 cm
Unit 16:   6 to 7 Months 
25 weeksGastrointestinal Intestinal lining contains all adult cell types
  Nervous Rods & cones
  Nervous The ability to taste
26 weeksSkin Additional fat deposits decrease wrinkles
  Skin Tear production
  Respiratory Terminal sac stage begins
  Nervous The ability to smell has arrived
26 weeks - 38 weeksSpecial Senses Brain weight increases 400% to 500%
27 weeksNervous Pupils react to light
28 weeksNervous Distinguishes sounds of different frequencies
  General Crown-heel length 39.5 cm
Unit 17:   7 to 8 Months 
30 weeksRespiratory Breathing motions are common even though there is no air in the womb
  Special Senses 6-layered cerebral cortex
  General Head circumference 30 cm
32 weeksMuscular Esophagus: Lower esophagus muscles functional
  Renal/Urinary Glomeruli formation complete
  Respiratory Alveoli
  Special Senses Memory - music preferences
  General Crown-heel length 45 cm
Unit 18:   8 to 9 Months 
32 weeks - 36 weeksNervous Prenatal food affects newborn taste preferences
34 weeksGeneral Rapid weight gain
35 weeksMuscular Firm grip
  General Amniotic fluid volume peaks
36 weeksRespiratory Surfactant production accelerates
  General Brain weight 300 grams
  General Crown-heel length 48.5 cm
Unit 19:   9 Months to Birth 
37 weeksGastrointestinal Fetus drinks an estimated 15 oz (or 450cc) of amniotic fluid/day
38 weeksRespiratory Air breathing begins
  Cardiovascular By term, the typical umbilical cord measures 20 to 24 inches (50 to 60 cm)
  Cardiovascular Heart beats 54 million times before birth
  Cardiovascular Major circulatory changes
  Special Senses Spinal cord ends at third lumbar vertebrae
  General Brain weight 350 grams
  General Crown-heel length 50 cm
  General Fetus initiates labor
  General Head circumference 35 cm
  General Time to be born!
66 weeks, 5 daysMuscular Premuscle cells form sheets representing muscles of facial expression