THE EARLY DEVELOPMENT OF THE CHICK 249 



The first step in the real process of gastrulation is the turning 

 under of a few of the marginal cells in this posterior region of 

 the blastoderm, where it is free from the germ wall. As the 

 involution of superficial cells continues, this margin soon 

 becomes thickened, and since the involuted cells are the rudi- 

 ment of the endoderm, the thickening represents the lip of the 



FIG. 92. Diagrams of reconstructions of the pigeon's blastoderm. From 

 Lillie (Development of the Chick) after Patterson. A. Thirty-one hours after 

 fertilization. B. Thirty-six hours after fertilization. C. Thirty-eight hours after 

 fertilization. E, Gut endoderm; GW, germ wall; O, margin of overgrowth; PA, 

 outer margin of area pellucida; R, in B, margin of involution; in C, mass of cells 

 left after closure of the blastopore; S, beginning of yolk-sac endoderm; SG, 

 anterior part of subgerminal cavity (blastoccel), as yet free from endoderm; Y, 

 yolk zone; Z, zone of junction. The numbers 1-7 along the line C-D, in A, 

 indicate the number of cells in the thickness of the blastoderm in these regions. 



blastopore, which is here reduced to a short crescent, between the 

 two free extremities of the germ wall (Fig. 92, B). Once 

 established, the endoderm rapidly grows forward between the 

 yolk and the ectoderm, as the upper cells of the blastoderm may 

 now be called, extending through the segmentation cavity 



