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). 



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. 



Mes 



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. 



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. These two portions of the mesoblast are often 

 termed gastral and prostomial, connected with the head-process 

 and primitive streak respectively. The head-process becomes 

 inseparably fused with the entoderm in the middle line imme- 

 diately after its formation; 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 separate again in the course of development. 

 But the anterior end of the notochord remains attached to the 



