Gastrulation in the Pigeon's Egg. 



99 



(Figs. 38 and 39). The position of the cavities suggests that they 

 are probably to be regarded as the remains of the cavity that was 

 formed between the upper and lower layers when the former turned 

 under to give rise to the latter. I might suggest that there is 

 another possibility, namely, that such cavities are the homologue of 

 "Kupffer's Vesicle." 



Fig. 40, which is from a section taken through the plane KF of 

 Fig. XI shows the tip of the right horn of the zone of junction 



Fig. XII. Reconstruction of a blastoderm taken thirty-six and one-fourth 

 hours after fertilization. Lettering is the same as in Fig. XI, FH, plane 

 of section represented in Figs. 20 and 21. The numbers within the areas 

 formed by the four intersecting lines indicate the number of degenerating 

 periblastic nuclei in these areas, x 27.2. 



and the lateral part of the dorsal lip. The length of the lip in sec- 

 tion becomes less and less in passing laterally, finally disappearing 

 altogether. Thus in Fig. 41 it is no longer present, and the margin 

 is occupied by the zone of junction, inside of which is a region whose 

 position would lead one to call it germ-wall, but it is probably more 

 correct to regard it as a portion of the lip that has already been 

 enclosed within the horns of the zone. Passing still farther to the 



