268 



ELEiVIENTS OF BIOLOGY 



the floor is formed by the large yolk cells of the vegetal hemi- 

 sphere. The pigmented cells of the animal hemisphere increase 

 in number and migrate downward over the vegetal hemisphere, 

 thus incorporating the yolk-containing vegetal hemisphere cells 

 within as the floor of the archenteron. The blastopore is then first 

 indicated by the crescentic shaped notch where the endoderm is first 

 turned inward. This is the dorsal lip of the blastopore, the lateral 



ANIMAL HEMISPHERE 



BLASTOCOELE 



ARCHENTERON 

 MESODERM 



DORSAL LIP OF 

 BLASTOPORE 



VOLK CELLS 



VEGETAL HEMISPHERE 

 Fig. 182. — Section through the gastrula stage of the frog embryo. 



and ventral lips being the line formed where the animal hemis- 

 phere cells are advancing over the vegetal hemisphere to converge 

 and form a circular blastopore under the dorsal lip. 



In the distinctly teleolecithal eggs of fishes, reptiles, and birds the 

 yolk-containing vegetal hemisphere does not cleave. Cell divisions 

 are at first confined to a region in the vicinity of the animal pole, 

 the cells forming a cap. Gastrulation is much modified. In birds it 

 is reduced to the inward growth of a wedge of cells, which forms 

 the endoderm by increasing in area by cell divisions and by addi- 

 tions of cells cleaved from nucleated cytoplasm that underlies the 



