BLASTULA, GASTRULA, AND GERM LAYERS 343 



tion and epiboly combined with delamination. The didermic 

 condition results, to a considerable extent, from the overgrowth 

 of the animal hemisphere cells (germ ring) which come to enclose 

 the yolk cells of the vegetal hemisphere. As in Amphioxus, 

 however, the yolk, although here so much more abundant, is 

 finally included in the floor of the gut cavity, and the yolk cells 

 take a direct share in the formation of the structures of the 

 later embryo. After the mesoderm and chorda have been 

 formed from the roof of the archenteron, this is left as a thin 

 layer, only one cell in thickness, quite in contrast with the thick 

 mass of cells forming its floor (Fig. 155). 



Turning now to the third type of gastrula, that formed from 

 the discoid blastula, we find conditions which vary widely from 

 the Amphioxus type, but which after all may be interpreted 

 in the light of the processes just outlined. In the Ganoid or 

 Amphibian, both the animal and vegetal hemispheres of the 

 egg share directly in the processes of cleavage, and blastula and 

 gastrula formation; and the yolk, contained in typical cells, is 

 carried directly into the wall of the primitive gut. But in 

 the extremely merolplastic eggs of the Elasmobranchs, Teleosts, 

 Reptiles, and Birds, the large yolk mass, which is the equivalent 



FIG. 157. Sagittal section through early gastrula of the catfish, Ameiurus. 

 en, endoderm; gr, germ ring; p, periblast; s, segmentation cavity, or sub-ger- 

 minal cavity; y, yolk. 



of the vegetal pole of the egg, does not cleave (Figs. 150, 48) 

 and takes no direct share in the formation of the cellular blastula 

 and gastrula. For comparative purposes, therefore, we have 

 already seen that we must recognize the germ disc or " blastula" 

 of this type as equivalent only to the animal hemisphere of such 

 a form as the frog or Amphioxus, flattened out and resting 

 upon the undivided yolk mass. In such a condition as this the 

 equivalent of the germ ring would be found forming the 



