98 



THE BIOLOGY OF THE FROC; 



CHAP. 



vertebral column, and occurring in the embryo, when not 

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



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

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

 chord ; E£, epidermic layer of ectoderm ; £/V, nervous layer of ecto- 

 derm ; Af, mesoderm ; Af£, outer or somatic mesoderm ; Af//, ini^er or 

 splanchnic mesoderm; jVC, neural gropve ; AID, dorsal root'of spinal 

 nerve; NS, spinal cord; J) archenteron ; W, liver diverticulum; 1^, 

 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 



