Gastrulation in the Pigeon's Egg. Y5 



gradually becomes less opaque (Fig. V, B). This eccentrically 

 lying region is the beginning of the area pellucida, and is brought 

 about by the development of the subgerminal cavity, together with the 

 thinning-out of that portion of the disc lying directly above this cav- 

 ity. At first the boundary between the areas opaca and pellucida is 

 very indistinct. In fact, this is more or less true throughout the entire 

 period of gastrulation, and it is not until just a few hours before the 

 egg is laid that a sharp differentiation between these two areas is 

 established — a condition characteristic of the nnincubated blasto- 

 derm. 



Within forty-five minutes after its appearance, the area pellucida 

 has practically doubled its diameter (Fig. V, C), this expansion 

 taking place most rapidly toward the posterior margin. During the 

 next two hours and a half the changes consist in an extension of 

 the processes just described (Fig. V, D-F). In some eases the area 

 pellucida extends almost to the posterior edge of the blastodisc, while 

 in others it is difficult to determine from surface views the exact 

 condition of this margin. Under high magniificatiou, however, the 

 posterior edge of the disc is seen to differ from the rest of the margin, 

 in that it does not blend into the surrounding yoIJv, hut eiuis rather 

 distinctly. 



At about thirty-four hours there occurs the most significant change 

 yet observed. It is the appearance of an indentation at the posterior 

 edge of the blastodisc. This bay is the beginning of the gastrula- 

 invagination , and often takes the form of a distinct marginal notch 

 (Fig. V, G). The edge of the disc included within the limits of 

 the bay is to be regarded as the dorsal lip of the blastopore, and, 

 owing to a slight opacity in this region, stands out in sharp contrast 

 to the rest of the margin. During the next half hour the blastoporic 

 margin changes from that of a uotch to that of a broad shallow bay 

 (Fig. V, 77), finally becoming straight (Fig. V, I). This straight 

 margin then becomes slightly rounded and less opaque (Fig. V, J), 

 and at the same time the rest of the blastodermic edge is sharply 

 defined. This change in the contour of the margin is duo to the 

 origin of the region of overgrowth, a structure that will be understood 

 better from a study of sections. 



