288 



COMPARATIVE ANATOMY 



group including obliquus and transversus sets. The muscles of the 

 diaphragm, which are peculiar to man and mammals, migrate into the 

 chest from the neck, as is evidenced by the fact that they are innervated 

 by branches of the third, fourth, and fifth cervical nerves. 



Tongue Muscles. The origin of the hypoglossal muscles in the human 

 embryo 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. 



SOMITES 10-14 



SOMITES 7-11 



IWTOMIC BUDS 

 VISCERAL ARCH 



A CYCLOSTO^E 



SOMITE \ MUSCLE 



'OLF/CTORY PIT VlSCERAU ARCH 4 

 B- ELASMOBRANCH 



HYPOGLOSSUS 



C^ REPTILE D. MAMMAL 



Fig. 240. — Diagrams illustrating the mode of origin of hypoglossal (hypobranchial) 

 muscles in A. Cyclostome, B. Elasmobranch, C. Reptile, and D. Mammal. In A, B, and 

 C cervical myotomes send myotomic buds into the hypobranchial region. In mammals 

 such buds are not formed but a migration of mesenchyme cells from cervical myotomes 

 provides material for these muscles. The number of myotomes which participate is 

 usually four or five. 



muscle buds grow from four to 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 

 hypoglossus. In man and mammals evidence is lacking of muscle buds 

 in the formation of the hypoglossus 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 with the trunk muscles, 

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

 the appendage, the appendicular muscle buds subdivide into dorsal and 



