ORIGIN OF THE SEX CELLS 



109 



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 



Multi plication 

 period 



'*;•" \M Primordial germ cell 



Oogonict 



\^ v^ ^^£7 "^^yy "^^^ M^N 

 /— \ / \ / \ / \ / \ I 



ancpl so on for oi large nurnber q-F cell divisions 



T\//^ (^ny one oocjoniuni 



Growth 

 period 



Maturation 

 period 



Secondary oocyte 



Matured 

 egg cell 



■Synapsis 

 Primary oocyte 



Meiot/c division 

 \ V^ Firs t polar b ody 



® m 



Polar bodies 



^Second polar 

 '■^ody 



■ '-Yolk accumulated in the 

 egg cell 



Fig. 42. — Diagram illustrating oogenesis, the ohromosome number being the same as in 



Fig. 41. 



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. 



