80 THE DEVELOPMENT OF THE CHICK 



the latter which corresponds to the future vascular area. The 

 mesoblast grows out, not only from the sides of the head-process 

 and primitive streak, but also from the hind end of the latter, 

 that is from the primitive plate. The mesoblast thus extends into 

 the opaque area behind the embryo at a very early stage (Figs. 

 42 and 44). This part of the mesoblast is homologous with the 

 mesoblast of the ventral lip of the blastopore of reptiles and 

 amphibia, and, like it, is the first place of formation of blood. 



The primitive groove must be regarded as an expression of 

 the forces of invagination of the mesoblast, and the primitive 

 folds as the lips of this invagination. 





.'■- "^^•-■ff* 





« oC Q ©• 



Fig. 41. — The part of the section shown in Fig. 40 C, between A-A and 

 B-B more highly magnified. 

 Abbreviations same as Fig-. 40. 



^fe' 



The Head-process. Two stages of the head-process are shown 

 in tranverse section a short distance in front of the primitive 

 knot in Figs. 39 A and 40 A. It consists of a thicker central 

 mass of cells with lateral wings; the central part, or primordium 

 of the notochord, is continuous posteriorly with the axis of the 

 primitive streak (Fig. 42); the lateral wings are mesoblast and 

 they are continuous posteriorly with the mesoblast wings of the 

 primitive streak. The head-process becomes inseparably fused 

 with the entoderm in the middle line immediate!}' after its forma- 

 tion; and this fusion is continued back along the axis of the 

 primitive streak (Figs. 39 and 40). The fusion is particularly 

 intimate and persistent at the extreme anterior end of the head- 

 process; behind this point the notochord and entoderm soon sepa- 

 rate again in the course of development. But the anterior end 



