THE DEVELOPMENT OF THE FROG 



97 



of mesoderm extend ventrally until they meet below and 

 come to surround the archenteron, except for a short space 

 along the dorsal side. The sheets of mesoderm soon be- 

 come split into an inner or splanchnic layer, which lies next 

 to the archenteron, and an outer, parietal, or somatic layer, 

 which lies next to the ectoderm. The space between these 

 two layers of mesoderm is the be -inning of the cos lorn, or 



FlG. 21. — Transverse section through the middle of a frog's embryo. CH, 

 notochord ; E, ectoderm ; M, mesoderm ; AG, neural groove ; NP, 

 neural plate ; T, mesenteron ; Y, yolk cells. (After Marshall.) 



body cavity. It is at first small, but as development proceeds, 

 it widens out more and more. 



The cells just above the mid-dorsal wall of the archenteron 

 form a thickening which soon becomes marked off sharply 

 from the mesodermic layers on either side and the wall of 

 the archenteron below. This thickening is the beginning 

 of the notochord, a structure forming the beginning of the 



