THE DEVELOPMENT OF THE FROG 267 



excretory systems. The formation of the mesoblastic somites, 

 or myotomes, has already been described. When some 

 fourteen of them are formed, the cells of the most anterior 

 myotomes begin to elongate in a longitudinal direction, each 

 cell becoming cylindrical in shape, and extending for the 

 whole length of the myotome. The process of differentiation 

 continues, and the myotomes are eventually transformed into 

 the trunk muscles. In the tail of the tadpole, the myotomes 

 are very conspicuous, and are bent, like those of Amphioxus, 

 into a V shape, with the angle directed towards the head 



(fig- 65). 



The lateral plates of mesoblast, as has been seen, are 



Fig. 65 



A tadpole of Ihe frog at the time of hatching. an, anus ; ex.g, external gills ; 

 na, nasal pit ; s, sucker ; som, somites ; st, stomoda;um ; yk, yolk-sac. 



divided, at an early stage, into a somatopleuric and a 

 splanchnopleuric layer. The former becomes closely at- 

 tached to the epiblast, and forms with it the body-wall. 

 The latter becomes attached to the wall of the gut, and 

 forms the musculature of the wall of the gut. The outer- 

 most cells of the splanchnopleur, and the inner cells of the 

 somatopleur — i.e. those lining the coelomic cavity^are con- 

 verted into the cceloraic epithelium, and in tadpoles Of about 

 ID mm. length, the first rudiments of the generative organs 

 are formed as two tracts of modified coelomic epithelium, 

 lying one on either side of the mesentery, on the dorsal wall 



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