212 CHORD ATE ANATOMY 



Tongue Muscles. The origin of the h\'poglossal muscles in the human 

 embrvo is somewhat uncertain. Since, however, they have the same 

 innervation as in lower vertebrates, it is generally assumed that their 

 development is essentially similar. In all vertebrates below mammals, 

 muscle buds grow from four or five occipital myotomes ventrally into 

 the floor of the throat. From the mass of cells thus formed arise the 

 intrinsic muscles of the tongue, innervated by the twelfth nerve, the 

 h>-poglossus. In man and mammals e\adence is lacking of muscle buds 

 in the formation of the hypoglossal muscles. It may be assumed that 

 cell migration takes the place of bud formation and extension. 



Appendicular Muscles, In the embryos of lower vertebrates, elasmo- 

 branchs to reptiles, as the myotomes grow ventrally in the body-wall 

 and reach the level of the lateral folds from which the appendages develop, 

 they give off lateral buds into the appendicular folds. After they have 

 entered the folds, these buds lose their connexion \\dth the trunk muscles, 

 although they still retain their epithelial character. Within the anlage of 

 the appendage, the appendicular muscle buds subdi\dde into dorsal and 

 ventral moieties, from which develop respectively the levator and depressor 

 muscles of the appendage. 



The appendicular muscles of man and mammals, on the contrary, 

 do not develop from myotomic buds, but arise by cell migration. The 

 two methods are after all not radically different. In fishes, for example, 

 where most of the appendicular muscles arise from myotomic buds, some 

 muscles which develop later than the others come from migrant mesen- 

 chymatous cells as they do in mammals. Similarity of innervation, 

 however, attests the homology of the appendicular muscles throughout 

 the vertebrate series. 



The fact that the arm muscles of man are innervated by the last four 

 cer\'ical and the first thoracic nerves further justifies the assumption that 

 they are derived from the myotomes of these segments. To the group 

 of muscles derived from this source, are added others, such as the trapezius, 

 sterno-cleido-mastoid, and levator scapulae. The pectoralis and latissi- 

 mus dorsi muscles spread out from the arm. Most of the muscles of the 

 shoulder, chest, and arm appear early in the second month, and are 

 differentiated by the beginning of the third. 



From the connexion of the muscles of the lower leg with spinal nerves, 

 including the last four lumbar and first three sacral, it may be assumed 

 that their cellular anlagen are derived from the corresponding myotomes. 

 In all essentials their development resembles that of the muscles of the 

 arm. A common mass of cells within the Hmb-bud differentiates into 

 dorsal and ventral muscle anlagen. The muscles from the ventral group 

 become innervated by the femoral nerve while the dorsal group are con- 

 nected with the obturator. The subdivision of the primary muscle 



