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Premuscle masses for the extraocular muscles appear dorsal to the optic cup. They receive the terminal fibers of cranial nerves III, IV or VI. Although there is phylogenetic evidence that three preotic somites give origin to the extraocular muscles, they never form in human embryos. These muscles apparently differentiate in situ from mesenchyme.


Mesenchymal cells within each branchial arch rapidly increase in quantity. Some begin to collect around the terminal portion of the branchial arch nerves and represent the premuscle masses from which the branchial musculature develops.

The sternocleidomastoid-trapezius premuscle mass appears just caudal to the cervical sinus and already receives the spinal accessory nerve. Its origin is controversial, but it is derived probably from the mesenchyme in the caudal arches and from the cervical myotomes.


Premuscle masses of the tongue are evident in the mesenchyme ventral to the pharynx and are closely associated with the termination of the hypoglossal nerve. Although there is phylogenetic evidence that the occipital myotomes give origin to the tongue muscles, there is no continuity in human embryos between them and the tongue premuscle masses. The tongue muscles probably differentiate in situ from the mesenchyme in the floor of the pharynx.


The occipital myotomes fuse into a single mass of myoblasts lateral to the proximal part of the hypoglossal nerve. The fate of this mass is unclear.

Caudal to the occipital region each myotome can be separated into a small, dorsal epiaxial division and a large ventrolateral hypaxial division. This is especially evident in the cervical region. The epiaxial divisions will give rise to the extensor muscles of the vertebral column in the back, which are innervated by the dorsal primary rami of spinal nerves. The hypaxial divisions will give rise to the lateral and ventral flexors of the vertebral column, which are innervated by the ventral primary rami of spinal nerves.


The somatic layer between the mesothelial lining of the coelomic cavity and the ectoderm becomes thicker but is without distinct premuscle masses.

The splanchnic layer thickens and condenses around the gut and tracheal endoderm. It will form the muscularis of the alimentary tract and the cartilages in the wall of the tracheobronchial tree.


Limb mesoderm becomes very dense especially near the base and in the central part of the limb bud. Condensations begin to appear near the terminal ends of the spinal nerves.

The musculocutaneous, median, ulnar and radial branches of the brachial plexus can be identified as they pass into the upper limb bud.

Mesoderm in the lower limb bud receives the terminal nerve fibers of the lumbar and sacral plexuses. Identification of specific nerves is more difficult.

Source: Atlas of Human Embryos.