THE MUSCULAR SYSTEM 



289 



ventral moieties, from which develop respectively the levator and depressor 

 muscles of the appendage. (Figs. 241, 242) 



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

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



SPINAL CORD 

 DERMATOME 

 SOMATIC MOTOR NERVE 

 MYOTOMr 



SENSORY GANGUON 



NOTOCHORD 



HYPOCHORDA 

 "DORSAL AORTA 



POSTCARD! NAL VEIN 



SPINAL NERVE 



FIN- BUD 



VITELUNE 

 VEIN 



MESENCHYME 



Fig. 241. — A cross section of a 17 mm. elasmobranch (squalus) embryo in the 

 trunk region, showing an early stage in the growth of a myotomic bud into the fin-fold. 

 The yolk-sac to which the embryo is attached has been removed. 



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, 



3ny 



my 



Fig. 242. — Budding of muscles of appendage from myotomes in PrisHurus. b, 

 muscle buds; my, myotomes. (From Kingsley's "Comparative Anatomy of 

 Vertebrates," after Rabl.) 



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 

 cervical and the first thoracic nerves further justifies the assumption that 



