9 8 



THE BIOLOGY OF THE FROG 



CHAP. 



vertebral column, and occurring in the embryo, when not 

 also present in the adults, of all vertebrate animals. It 



NO 



EE 



NS 



CH 



M 



FIG. 22. Transverse section through a frog embryo before the closure of 

 the medullary or neural folds. C, ccelom or body cavity; CH, noto- 

 chord ; EE, epidermic layer of ectoderm ; E\, nervous layer of ecto- 

 derm ; M, mesoderm ; ME, outer or somatic mesoderm ; MH, inner or 

 splanchnic mesoderm; JVC, neural groove; ND, dorsal root of spinal 

 nerve ; NS, spinal cord ; T, archenteron ; W, liver diverticulum ; Y, 

 yolk. (After Marshall.) 



is always the first part of the skeleton to make its appear- 

 ance in the embryo, as it was the first part to appear in the 

 evolution of the race. Whether in the frog it is entodermic 



