The extrinsic ocular muscles will form in the mesenchymal tissue dorsal to the optic cup. A series of muscles and supporting structures will differentiate in the mesodermal core of the branchial arches. The muscles of the tongue will develop in the mesenchyme in the floor of the pharynx.
SOMITIC (PARAXIAL) MESODERM
Up to 42 to 44 somites differentiate on each side of the neural tube from the rhombencephalon to the coccygeal or tail region. There are four occipital, eight cervical, twelve thoracic, five lumbar, five sacral and eight to ten coccygeal. The first occipital somite disappears. The last five to seven coccygeal somites will disappear.
Cells in the ventral and medial walls of the somites actively proliferate and migrate toward the neural tube and notochord to form the sclerotome in each segment. This activity begins in the more cranial somites and proceeds in a caudal direction. Each sclerotome is separated by a dorsal intersegmental artery.
Cells in the dorsal wall of the somite retain their epitheloid character a while longer and compose the dermatome that is adjacent to the single-layered ectoderm.
Lightly staining, spindle shaped cells called myoblasts collect along the medial side of the dermatome. These cells form a platelike structure in each segment called the myotome, which will give rise to skeletal muscles.
INTERMEDIATE MESODERM
From the intermediate mesoderm will come most of the components of the Urogenital System.
LATERAL (PLATE) MESODERM
The lateral mesoderm is separated by the coelom into somatic and splanchnic layers each of which becomes more cellular.
The somatic layer contributes to the formation of the ventro-lateral body wall.
The splanchnic layer surrounds the primitive gut. Condensed, splanchnic mesoderm in the dorsal mesogastrium will form the splenic primordium.
LIMB MESODERM
The limb buds contain dense mesoderm surrounded by thickened ectoderm or placode. The bones and muscles of the extremities form within this mesoderm.
Various studies on lower forms suggest three possible origins for this tissue: a) somites, b) lateral mesoderm or c) placode.
The upper limb bud forms at the level of neural crests C-5 to T-1. The lower limb bud forms at the level of neural crests L-1 to S-2. These segments are retained in the definitive condition and correspond to the segments of the motor and sensory nerves of the extremities.
Source: Atlas of Human Embryos.