60 



EMBRYOLOGY. 



The second plane of division is consequently, like the first, vertical, 

 and cuts the latter at right angles. 



After the conclusion of the second segmentation the Amphibian 

 egg consists of four quadrants (fig. 32 4 ), which are separated from 

 one another by vertical planes of division and possess two dissimilar 

 poles, one richer in protoplasm, lighter, and directed upwards; the 

 other richer in yolk, heavier, and directed downwards. In the case of 

 equal segmentation we saw that at the stage of the third segmentation 

 the axis of the nuclear spindle becomes parallel to the long axis of 

 the quadrant. The same thing occurs here also, although in a some- 

 what modified manner. On account of the greater accumulation of 

 protoplasm in the upper half of the quadrant, the spindle cannot, as 



fig. 32. Cleavage of Rana temporaria, after ECKER. 



The numbers placed above the figures indicate the number of segments present in the corre- 

 ponding stage. 



in the case of equal segmentation, lie in the middle of it, but must 

 lie nearer to the animal pole of the egg (fig. 31 B sp). Moreover, it 

 is exactly vertical, because the four quadrants of the Amphibian egg 

 are definitely oriented in space on account of the difference in specific 

 gravity of their halves. In consequence of this the third plane 

 of division -must be horizontal, and must also lie above the equator of 

 the egg-sphere more or less toward its animal pole (fig. 32 8 ). The 

 segments are very unlike both in size and composition ; and this is 

 the reason why this form of segmentation has been called unequal. 

 The four upper segments are smaller and contain less yolk, the four 

 lower ones are much larger and richer in yolk. They are also 

 distinguished from each other as animal cells and vegetative cells, 

 according to the poles near which they lie. 



In the course of further development, the distinction between, 

 animal and vegetative cells constantly increases, for the richer_the 

 cells are in protoplasm the more quickly and the more frequently 



