54 TEXT-BOOK OF EMBRYOLOGY. 



over the ventral lip to form the floor. The imagination cleft which thus be- 

 comes the archenteron is at first small as compared with the segmentation cavity 

 but rapidly increases in size, until as in Amphioxus, the earlier cavity is finally 

 completely obliterated (Fig. 37). Coincident with the carrying of the yolk 

 cells into the interior of the vesicle and the obliteration of the segmentation 

 cavity, proliferation of the micromeres carries them completely around the yolk 

 cells, so that the entire surface of the gastrula is formed of small cells (Fig. 37). 

 The amphibian gastrula thus consists of a central cavity, the archenteron, 

 communicating with the exterior by means of a small opening, the blastopore, 

 the roof of the cavity being formed by two or more layers of small cells, the 

 floor by the mass of large yolk cells. The outer layer of cells completely sur- 

 rounds the yolk cells except at the blastopore, and constitutes the ectoderm 

 (Fig. 37). The inner layer or entoderm is distinct only in the roof of the cavity. 

 Laterally its cells pass over without any distinct demarcation into the mass of 



Ectoderm 



Entoderm (protentoderm) 



Archenteron 



Yolk cells (yolk entoderm) 

 Peristomal mesoderm 



Yolk plug 



Posterior lip of blastopore 



Peristomal 

 mesoderm 



FIG. 37. Vertical section through gastrula of Triton. Hertwlg. 



yolk cells which form the floor of the cavity. As the ectoderm forms a com- 

 plete outer layer, the only point at which the yolk cells now appear externally is 

 the blastopore, into which they project as the yolk plug (Fig. 37). 



It is possible in the amphibian gastrula to make the distinction between the 

 entoderm of the roof which has grown in from the surface and is continuous 

 with the surface ectoderm, and the entoderm of the floor which is formed of yolk 

 cells. By those who make this distinction, the former is called the protentoderm t 

 the latter the yolk entoderm (Fig. 37). 



In the case of the common frog, the eggs of which are so easily obtained 

 that they furnish most satisfactory subjects for study, gastrulation is somewhat 

 less simple than in Triton. As already noted (p. 47) the demarcation between 

 micromeres and macromeres is in the frog very distinct, owing to the dark pig- 

 mentation of the former. This is shown in Fig. 30, as is also the fact that the 

 roof of the segmentation cavity consists of a surface layer of strongly pig- 



