Development of the First Day 129 



ectoblast has become thickened, in a line above 

 the developing notochord, to form the medul- 

 lary plate. The sides of the medullary plate 



St 



FIG. 44. SURFACE VIEW OF EMBRYO AT THE TWENTY-THIRD 

 HOUR OF INCUBATION. (After Duval.) 



A , anterior limit of head. /*/, primitive streak. ^z>, mesoblastic somites. 

 St, 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 mediillary groove between them (Figs. 

 43 and 44). The medullary folds, before the 



