ADAPTATION IN CLEAVAGE. 



57 



the metaphase of the first maturation-spindle, and the sperm- 

 nucleus remains absolutely quiescent during the remainder of 







Fig. 12. — Early fertilization of the egg of Unio complaiiata in actual section. The space be- 

 tween the membrane and the egg proper has been reduced to economize space ; the micro- 

 pyle, penetration-path of the spermatozoon, sperm-amphiaster, and first maturation-spindle 

 are shown. A large proportion of the yolk-granules is temporarily forced to the periphery. 



Fig. 13. — Formation of the second polar globule in U7tw. Sperm-nucleus to the left. 



the maturation processes, until the egg-nucleus begins to grow. 

 Let us now study the q%% during the final constriction off of 



