80 



EVOLUTION AND GENETICS 



Fig, 34. — Diagram illustrating the two maturation divisions of the 

 germ cells in the male. In a the chromosomes appear as thin threads 

 (leptotene stage). These conjugate in pairs, b, beginning at the two 

 ends of each loop. The threads contract, and a spindle appears, d, near 

 the nucleus. The conjugating chromosomes enter the spindle, d. There 

 they separate, e, moving to opposite poles of the spindle. The cell pro- 

 toplasm begins to constrict, /. The chromosomes may without entering 

 upon a resting nuclear stage pass onto a new spindle that has de- 

 veloped by the division of each of the centrosomes of each daughter 

 cell, g. Each chromosome now splits throughout its length (equational 

 division) ; half of each goes to one or the other pole. The two daughter 

 cells then divide, giving four cells, each of which differentiates into a 

 spermatozoon. 



