THE DEVELOPMENT OF THE MUSCULAR SYSTEM. 



273 



In the first place among the lower Vertebrates the muscles of each extremity are 

 derived from several myotomes and are innervated by segmental nerves cor- 

 responding to these myotomes. In the second place among the higher Verte- 

 brates, although the myotomic origin of the muscles has not been clearly demon- 

 strated, the nerve supply in each extremity comes through several segmental 

 spinal nerves. 



Knowledge concerning the development of the individual muscles of the ex- 

 tremities in the human embryo is incomplete. Especially is this true of the 

 muscles of the lower extremities. 



The upper limb bud first appears in embryos of 2-3 mm. (during the third 

 week) as a slight swelling ventro-lateral to the myotomes in the lower cervical 



Upper limb bud 



Border vein 



M& Somatopleure 



FIG. 232. Transverse section through the eighth cervical segment of a human 

 embryo of 4.5 mm. Lewis. 



region (Fig. 231; see also Fig. 87). The swelling gradually enlarges and by 

 the time the embryo has reached a length of 4-5 mm. lies opposite the last four 

 cervical and the first thoracic myotomes. At this time it is filled with closely 

 packed mesenchymal cells. No buds from the myotomes can be seen extending 

 into the mesenchyme (Fig. 232). 



In succeeding stages the limb bud enlarges still more, and the mesenchymal 

 tissue becomes denser (Figs. 233 and 234). During these stages no growths, 

 either of buds or of individual cells, from the myotomes are apparent. Some 

 of the cervical nerves, however, enter the limb buds (Fig. 234). 



Apparently the tissue from which the muscles, as well as the skeletal ele- 

 ments, are to develop, is the condensed mesenchymal tissue. The first indica- 

 tion of differentiation occurs during the fourth week (embryo of about 8 mm.). 

 The central portion or core of the mesenchymal mass becomes still denser to 

 form the anlage of the skeletal elements of the extremity. The tissue of the 



