ONTOGENESIS 155 



in the production of four cells of equal size, each of 

 which metamorphoses into a sperm-cell character- 

 istic of its species and capable of " fertilizing " an 

 egg-cell. But in the oocyte of the first order, whereas 

 the cleavage of the nucleus is equal in each of the 

 two daughter-cells formed, that of the cytoplasm is 

 so unequal that one of the daughter-cells is but a 

 fraction of the size of the other. In the second 

 cleavage the same 'disparity is seen, so that, al- 

 though as the result of the two successive cell divi- 

 sions four cells result, one of these is very many 

 times the bulk of the other three together. This 

 larger cell is the one usually called the egg. The 

 other three cells, which may be looked upon as 

 abortive eggs, are called polar bodies because they 

 usually remain attached for a time to the so-called 

 animal pole of the egg-cell. They finally fall off 

 and disintegrate. Like the sperm, the egg which 

 has budded off its polar bodies has the reduced 

 number (one half) of chromosomes. 



NUCLEAR PHENOMENA OF ZYGOSIS IN ANIMALS 



Conjugation. The male gamete, or sperm, is, 

 in most animals and in the lower aquatic plants, a 

 highly specialized cell of minute size, equipped with 

 one or more whiplike flagella that enable it to swim 

 rapidly in a liquid medium. In the higher animals 

 the shape of the sperm is often that of a miniature 

 tadpole. The egg-cell, on the other hand, in most 

 cases is spherical in form. It is enormously greater 

 in volume than the sperm, its size depending upon 



