GERM LAYERS. 57 



as well as of Reptiles and Birds, is essential to a proper appreciation of the 

 process in Mammals. 



Recalling the amphibian blastula (p. 50), it will be remembered that its 

 roof was formed of smaller protoplasmic cells (micromeres) while its floor con- 

 sisted of a mass of yolk cells which encroached upon the segmentation cavity 



Micromeres 



Macromeres 



FIG. 35. Vertical section through blastula of Triton. Hertwig. 



(Fig. 30). The zone of union between the two kinds of cells is known as the 

 marginal zone. The simplest type of amphibian gastrulation, and the type 

 thus most easily compared with gastrulation in Amphioxus, is exemplified by 

 the water salamander Triton taeniatus. (Compare Figs. 34 and 35.) 



Ectoderm 

 Entoderm 



Anterior lip of blastopore 



Blastopore 



Posterior lip of blastopore 



Yolk cells 

 (entoderm) 



Segmenta- __ 

 tion cavity 



FIG. 36. Vertical section through embryo of Triton, showing beginning of gastrulation. Hertwig. 



In Triton, a slight groove or furrow appearing along a portion of the marginal 

 zone marks the blastopore and the beginning of gastrulation. The upper lip 

 of this groove is formed by the smaller protoplasmic cells, the lower by the large 

 yolk cells (Fig. 36). The groove next deepens, the micromeres growing in at 

 the dorsal lip to form the roof of the archenteron, while the yolk cells are carried 



