SEGMENTATION 



35 



(Fig. 15). The spindles for the third cleavage are located near the animal pole and the cleavage 

 takes place in a horizontal plane. As a result, the upper four cells are much smaller than the 

 lower four. The large yolk-laden cells divide more slowly than the upper small cells. At the 

 blastula stage, the cavity is small, and the cells of the vegetal pole are each many times larger 

 than those at the animal pole. The cleavage of the frog's ovum is thus complete but unequal. 



4 5 



Fig. 15.— Segmentation of the frog's ovum (Hatschek in Marshall). B, segmentation cavity; 



U, nucleus. 



Ova of Reptiles and Birds. — The ova of these vertebrates contain a large 

 amount of yolk. There is very little pure cytoplasm except at the animal pole 



