i3 2 Vertebrate Embryology 



day, a slight crescentic fold is seen in the 

 area pellucida just in front of the head-fold 

 (Fig. 44, A) ; this is the beginning of the 

 amnion, but a description of its development, 

 in addition to what has already been given, 

 will be given at a later place. 



One of the most important changes of the 

 latter part of the first day has to do with the 

 mesoblast. After it has become fully estab- 

 lished, the mesoblast on each side of the 

 notochord forms (as seen in cross section) 

 a sort of wedge-shaped sheet of tissue, with 

 the base of the wedge next to the notochord. 

 At about the twenty-first hour, each of these 

 sheets of mesoblast splits into two layers 

 (similar to the cleavage of the mesoblast seen 

 in the frog), the upper layer being known as 

 the somatopleure, the lower layer the splanch- 

 nopleure. The space between the two layers 

 forming, as in the frog, the body-cavity or 

 coelom. The somatic layer becomes closely 

 associated with the ectoblast and forms the 

 body-wall ; while the splanchnic layer, to- 

 gether with the entoblast, forms the wall of 

 the digestive tract (Fig. 52). 



At about the twenty-second hour, almost im- 

 mediately after its cleavage, the mesoblast on 



