ORIGIN OF THE SEX CELLS 



111 



the cytoplasm. The nucleus becomes the larger part of the body, or 

 head, of the mature sperm cell, which in many higher animals resem- 

 bles in shape a tadpole with a very long tail. From a portion of the 



^^^^^Xm Primordial germ celt 



Multiplicoition 

 period 



and so on for a larye f^^J^^^er of cell d/i^/s/ons 



^FyT^ any one oocjonium 



Growth 

 period 



^-'Synapsis 



Primary oocyte 



Moif urcation 

 period 



Secondary oocyte 



Matured 

 egg cell 



C/ \ Me /otic division 

 1 (i^ First polar body 



® ® 



Polar bodies 



^Second polar 

 hody 



°\ ,''Yolk accumulated in the 

 egg cell 



Fig. 45. — Diagram illustrating oogenesis, the chromosome number being the same as in 



Fig. 44. 



cytoplasm is formed the tail, and between the head and tail, connecting 

 one to the other, is a middle piece which contains the centrioles. This 

 tail, by rapid vibrating movements, can propel the sperm cell through a 

 hquid medium at a relatively high rate of speed. 



