146 



THE EVOLUTION OF SEX. 



undergo a marked modihcation, the same in Doth cases. Round the chro- 

 matin rods vacuoles are formed, limiting them from the surrounding 

 protoplasm ; into these the rods send out anastomosing processes, after the 

 fashion of little rhizopods ; gradually the rods thus resolve themselves into 

 a network, in the meshes of w hich minute " nucleoli " are also demonstrable. 



,1 ryJ^ 



w 



'T> 



/-^ 



w 



the Process of Fertilisation, iaileaA^ng Bo\eri«» fi^juro — ,/ female pro- 

 nucleus ; (5, polar bodies ; c, male nucleus ; d, sperm-cap ; ac, chromatin elements 

 of uniting united female and male nuclei (a and c) ; e, protoplasmic centres ; y, 

 archoplasmic threads. 



The two nuclei thus modified then unite, but that again so precisely, as 

 Van Beneden especially has shown, that each forms half of that spindle 

 figure which almost all nuclei take when about to divide. This double 

 spindle figure is the "segmentation nucleus,"' which will presently divide 

 into the two first daughter-nuclei of the ovum (see figs. VI. -X.). 



It is not possible here to discuss certain intricate changes which take 

 place meanwhile, not in the nuclei, but in the cell-substance of the ovum. 

 Both Van Beneden and Boveri have recently agreed on the existence of two 

 "central corpuscles" (centrosomata) in the protoplasm, which serve as 

 " points of insertion " for protoplasmic threads, which exert a "muscular 

 action '"'■ upon the nuclear elements in the forthcoming division. Boveri 



