124 Vertebrate Embryology 



is caused by an elongated group of cells that 

 have been proliferated off from the ectoblast, 

 and now lie between the ectoblast and ento- 

 blast. These cells gradually spread out on each 

 side of the primitive streak, and form a part 

 of the mesoblastic layer. 



The primitive streak, for a time, keeps pace 



BEV Tiis 



Fig. 41. — Median portion of a transverse sec- 

 tion OF AN EMBRYO AT THE TIME OF THE FORMATION 

 OF THE PRIMITIVE STREAK. (After Duval.) 



eg; subgerminal cavity, ex, ectoderm, in, entoderm, 

 wj, mesoderm. /'/', primitive groove. 



in growth with the area pellucida, and as that 

 area grows more rapidly at its posterior end, 

 the primitive streak increases in length almost 

 entirely from its hinder end. 



A median furrow, varying somewhat in 

 width and depth, in different embryos, makes 

 its appearance in the upper surface of the 

 primitive streak (Figs. 40 and 41 //), and is 

 known as \\-\^ primitive groove. 



