REPRODUCTION AND DEVELOPMENT 



267 



ANIMAL POLE 



VEGETAL POLE 



Fig. 180. — An unseg- 



mented frog egg. 



ANIMAL HEMISPHERE 



Gastrulation in the Frog Embrj^o. Examination of the un- 

 divided fertilized t^^ of the frog shows that it may be roughly 

 divided into two hemispheres, one darkly 

 pigmented, the other a light yellow (Fig. 

 180). The yolk content of the yellow hemi- 

 sphere is much greater than in the pigmented 

 hemisphere. This is known as the vegetal 

 HEMISPHERE and its geometrical pole is called 

 the VEGETAL POLE. The black hemisphere is 

 the ANI^L\L HEMISPHERE, its pole being the 

 ANIMAL POLE. The presence of yolk appears 

 to slow up cell divisions, so when the fertil- 

 ized ^^g of the frog divides, the division 

 planes start at the animal pole and first separate the animal hemi- 

 sphere. Internally in such eggs the blastocoele is dislocated toward 



the animal pole, being partially 

 filled by the large, slowly divid- 

 ing cells of the vegetal hemi- 

 sphere (Fig. 181). A frog blas- 

 tula then consists of cells of 

 different sizes, the smaller ac- 

 tively dividing cells of the ani- 

 mal hemisphere and the large, 

 slowly dividing, yolk-contain- 

 ing cells of the vegetal hemi- 

 sphere. 



Gastrulation is also affected 

 by the amount and distribution 

 of the yolk. Gastrulation in the 

 frog blastula begins as a cres- 

 centic shaped notch near the 

 equator between the animal and vegetal poles (Fig. 182). Here 

 some cells are tucked in to form the roof of the archenteron; 



VEGETAL HEMISPHERE 



Fig. 181. — Section through the blastula of 



the frog embryo. 



