98 



THE BIOLOGY OF THE FROG 



vertebral column, and occurring in the embryo, when not 

 also present in the adults, of all vertebi..te animals. It 



NO 



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

 tlie medullary or neural folds. C, coelom or body cavity ; CH, noto- 

 chord ; EE, epidermic layer of ectoderm ; EN, nervous layer of ecto- 

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

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

 nerve; NS, spinal cord; j; 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 froi^ it is entodermic 



