THE EARLY DEVELOPMENT OF THE CHICK 251 



margin of the blastoderm resting upon the germ wall and fusing 

 peripherally with it. Posteriorly, where the germ wall is nar- 

 rower, there is a thickened region of the cellular germ disc which 

 represents the contracted blastoporal margin. In this region 

 the ectoderm and endoderm are continuous. 



During the first few hours of incubation, or even before lay- 

 ing in cases where the eggs have been retained for some time 

 longer than usual in the vagina, the endoderm extends com- 

 pletely across the segmentation cavity and becomes organized 

 into a fairly definite layer. 



From the preceding description it will be seen that the 

 process of gastrulation in the chick is essentially a process of 

 involution. There is no true invagination, and the process of 

 epiboly is not immediately concerned in the establishment of 

 the primary inner layer. Moreover, the process of epiboly is 

 here greatly limited, being restricted to a narrow posterior 

 section of the blastoderm. In other words, the entire blasto- 

 poral region is greatly reduced, doubtless in correlation with the 

 excessive amount of yolk in the ovum. 



The formation of the middle germ layer and the chief axial 

 structures of the embryo, is not intimately bound up with the 

 gastrulation process, as in the forms previously described. We 

 may now consider these processes in connection with the gen- 

 eral development of the whole embryonic region. 



II. THE FORMATION OF THE EMBRYO 



The earliest indication of the true embryo becomes visible 

 during the early hours of incubation, immediately after the 

 entire area pellucida has become two layered, through the com- 

 plete extension of the endoderm. It appears first as a slightly 

 thickened band, not very well marked, extending directly from 

 a point approximately in the middle of the area pellucida, 

 nearly to its posterior margin. This is called the primitive 

 streak; it is constituted at first solely by a thickening in the 

 ectodermal layer (Fig. 93). Once established, the primitive 

 streak grows very rapidly, chiefly through posterior elongation, 



