SO 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. 
Fria. 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 (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 immediately 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 
