76 THE DEVELOPMENT OF THE CHICK 



of the chick at a more advanced stage, but before the formation 

 of the head-process. Sections in front of the primitive streak 

 show no cells between ectoderm and entoderm. In the region 

 of the primitive knot (A) the ectoderm is greatly thickened, 

 forming a projection above and below. Cells become detached 

 from the lower surface of the ectoderm, and are converted into 

 migratory cells between the two primary layers. Immediately 

 behind the primitive knot the primitive groove begins abruptly; 

 it is the seat of active proliferation from the lower layer of the 

 ectoderm, and the cells migrate out laterally forming wings of 

 cells, which do not, however, reach the area opaca (Fig. 38 B). 

 Conditions are very similar along the entire length of the primitive 

 streak at this time; but near the posterior end a few cells of the 

 mesoderm reach the area opaca and begin to insinuate themselves 

 between the ectoderm and the germ-wall. There is no evidence 

 at any place that any of the mesoderm cells are derived from the 

 entoderm. The axial thickening of the primitive groove comes in 

 contact with the entoderm and appears in places fused to it. 



Figures 39 A-E represent five sections through the head-process 

 and primitive streak of a chick embryo at a time when the head- 

 process is still very short. The first section through the head- 

 process is described beyond. B is through the primitive knot; 

 the ingrowth of cells is more extensive than in the preceding 

 stage and it will be observed that they are now fused with the 

 entoderm, so that the latter no longer appears as a distinct layer. 

 C is through the primitive groove near its anterior end. D is a 

 little behind the center of the primitive groove, and E is through 

 the primitive plate. Behind the center of the primitive streak 

 the entoderm is again free (D). It will be observed that the 

 area of proliferation in the primitive plate is very wide. 



FIG. 39. Five sections through the head-process and primitive streak of a 

 chick embryo. The head-process is very short. 



A. Through the head-process, now fused to the entoderm. 



B. Through the primitive knot. 



C. Through the anterior end of the primitive groove. 



D. A little behind the center of the primitive streak. 



E. Through the primitive plate. 



The total number of sections through the head-process and primitive 

 streak of this series is 102. B. is 4 sections behind A. C. is 12 sections behind 

 A. D. is 59 sections behind A. E. is 87 sections behind A. 



Ect., Ectoderm. Ent., Entoderm. G. W., Germ-wall. H. Pr., Head- 

 process, med. pi., Medullary plate. Mes. Mesoblast. pr. f. Primitive fold, 

 pr. gr,, Primitive groove, pr. kn., Primitive knot. pr. pi., Primitive plate. 



