Development of the First Day 1 29 



ectoblast has become thickened, in a line above 

 the developing notochord, to form the medul- 

 lary plate. The sides of the medullary plate 



Fig. 44. — Surface view of embryo at the twenty-third 

 HOUR OF incubation. (After Duval.) 



A , anterior limit of head, pp^ primitive streak, /w, mesoblastic somites. 

 Si. sinus terminalis, bounding the vascular area, i, region where the medullary 

 folds have almost met to form the medullary canal. 



soon become elevated as the medullary folds, 

 with the m.edullary groove between them (Figs. 

 43 and 44). The medullary folds, before the 



