THE EARLY DEVELOPMENT OF THE CHICK 263 



germ wall, where it is continuous with the vascular area. The 

 inner or axial border of each sheet is considerably thickened, and 

 often distinguished as the vertebral plate, the remaining distal 

 portion being known as the lateral plate. The mesoderm also 



FIG. 100. Chick embryo of about twenty hours, showing first intersomitic 

 furrow. Dorsal view. From Lillie (Development of the Chick).- a.c.v., Amnio- 

 cardiac vesicle; a.o., inner margin of area opaca; Ect., ectoderm; Ent. endoderm, 

 H.F., head-fold; i.s.f.l., first intersomitic furrow; med.pl., medullary plate; Mes., 

 mesoderm; n.gr., neural groove; Pr'a., proamnion; pr.gr., primitive groove. 



extends posteriorly from the primitive streak, but at this time 

 I there is none in the anterior part of the blastoderm in front of 

 the head process and head-fold (proamnion), although around 

 the margin of the pellucid area it is carried forward as a pair of 

 [wing-like extensions, so that in surface view its anterior limit is 

 larkedly concave (Fig. 93, D). 



