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TEXT-BOOK OF EMBRYOLOGY. 



of the segments form the muscle plates or myotomes (Fig. 261), from which 

 develop by far the greater part, at least, of the voluntary striated muscles. 



The differentiation of the parts of the primitive segments begins in the cervi- 

 cal region by the end of the second week, and then gradually proceeds toward 

 the tail. Three myotomes are also probably formed in the occipital region. 

 The cells of the myotomes are at first of an epithelial character (Fig. 143). 

 Contractile fibrils appear in the cells and the latter are transformed directly 

 into muscle fibers. (For histogenesis see p. 307). The fibers later alter their 

 direction in accordance with the particular muscle to which they belong. The 

 muscle tissue first formed is thus segmented, being derived from the segmen- 



Pronephros 



Parietal mesoderm* 



Fig. 261. — Transverse section of human embryo of the 3rd week. Scl. 1 , Break in myotome at 

 point where sclerotome is closely attached. Kallmann. 



tally arranged myotomes, but as development proceeds the myotomes undergo 

 extensive changes by which the segmental character is lost in the majority of 

 cases. It is retained, however, in a few instances, such for example as the 

 intercostal muscles. The course of the changes which obliterate the segmental 

 character of the myotomes and give rise to the various muscles has not been 

 observed in all cases. But since a nerve belonging to any particular segment 

 and innervating the myotome of that segment always innervates the muscles 

 derived from that myotome, it is possible to learn something of the history of 

 the myotomes by studying the innervation of the muscles. 



From a consideration of what is known concerning the individual histories 



