GASTRULATION IN THE PIGEON'S EGG. 26 1 



characterized by a fusion of layers, in which there may be a 

 more or less syncytial condition. Above the region of the inner 

 or yolk zone a distinct ectoderm is present, below which are found 

 many large cells heavily laden with yolk. These cells, which 

 may be more or less surrounded by yolk, in which numerous 

 " periblastic " nuclei are present, are destined to become the yolk- 

 sac entoderm. This zone is the region formerly occupied by the 

 zone of junction. 



Within the subgerminal cavity are important structures. Scat- 

 tered over the greater part of its area, but more numerous in the 

 anterior region, are many large yolk masses, among which are 

 also a few of the remaining segmentation cells that have not yet 

 succeeded in getting into the ectoderm. At the extreme sides of 

 the cavity are a great number of cells (S), which in the main are 

 the same as the latter. However, it is possible that some of these 

 may have been given off from the inner edge of the germ-wall. 

 E, a tongue-like process, is the region over which the invaginated 

 entoderm extends. It reaches from near the posterior margin to 

 slightly beyond the center of the blastoderm. Its anterior and 

 antero-lateral margins end freely, but its postero-lateral margins 

 are bounded by the horns of the germ-wall. At the extreme 

 posterior the entoderm is in connection with the thickened rim, 

 a region of indifferent structure. Where the entoderm arises 

 from the rim it is necessarily thick but it gradually thins out an- 

 teriorly. Beneath this rim (R) is a passage, the blastopore, which 

 is widest at the margin, gradually narrowing as it passes toward 

 the center. This passage becomes continuous with the cavity 

 under the entoderm — the archenteric cavity. 



Sections of this blastoderm are very instructive. A portion 

 of the anterior half of a section, four sections to the left of the 

 median line, is shown in Fig. 7. At the extreme anterior margin 

 is the region of overgrowth (0), where the blastoderm first 

 spreads over the yolk. The width of the overgrowth never 

 exceeds that shown in this series, for as fast as it extends out 

 over the yolk the germ-wall keeps pace with it. In the more 

 peripheral portion of the germ-wall separate layers cannot be 

 distinguished. This is the zone of junction, which seldom has 

 a greater width than is shown in this figure. But next to the 



