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The optic vesicle folds in on itself to become a double-layered optic cup surrounded by loose mesenchyme. The intraretinal space separates the inner and outer layers.

The outer layer is thin and becomes the pigmented layer of the retina.

The thick inner layer surrounds the optic cup cavity and becomes the visual layer of the retina. It differentiates into three zones that are similar to the neural tube wall called the ependymal, mantle and marginal layers. The deepest zone is the ependymal layer, which is separated from the pigmented layer of the retina by the intraretinal space.

The ventral surface of the optic cup and adjacent stalk folds inward to form the choroid fissure through which loose mesenchyme passes into the cup cavity.


The lens placode invaginates to form the hollow lens vesicle and lens cavity. The lens vesicle loses contact with the ectoderm and lies within the cavity of the optic cup. The overlying ectoderm will contribute to the formation of the cornea.

The epithelial cells in the deep wall of the lens vesicle (deep lens epithelium) lengthen and project into the lens cavity. They will give rise to the entire lens with the exception of the superficial (anterior) lens epithelium that develops from the superficial wall.


Premuscle masses for the extraocular muscles become evident dorsal to the optic cup and receive the terminal fibers of cranial nerves III, IV or VI.



The medial wall of the otocyst evaginates dorsally to produce the endolymphatic diverticulum. The diverticulum terminates near the roof of the fourth ventricle.

The otocyst constricts into a large dorsal portion called the vestibular pouch and a narrow, tapered ventral portion called the cochlear pouch. The lateral groove that appears on the lateral surface of the vestibular pouch demarcates the posterior semicircular canal.

The acoustic part of the facioacoustic neural crest becomes the vestibulocochlear nerve (VIII). Ganglion cells begin to differentiate in the area of contact between the nerve and the otocyst. The vestibular ganglion forms adjacent to the vestibular pouch and the cochlear (spiral) ganglion develops adjacent to the cochlear pouch.

Mesenchymal cells begin to accumulate around the otocyst forming the otic capsule condensation.


The first pharyngeal pouch extends dorsally as the tubotympanic recess and makes contact with the first branchial groove. The tissue in the area of contact represents the primitive tympanic membrane.

The first (Meckel’s) and second (Reichert’s) arch cartilages are present as precartilage condensations medial to the mandibular and facial nerves, respectively. The lateralmost extent of the tubotympanic recess lies between these nerves. The geniculate ganglion of the facial nerve differentiates just above the recess and the chorda tympani branch passes just below.


Small elevations called auricular hillocks become evident on the surface of the first and second branchial arches adjacent to the first branchial groove. Three appear on each arch.

Source: Atlas of Human Embryos.