SEXUAL REPRODUCTION. 



J 37 



simultaneous change, which excludes other male elements. Only the head or 

 nuclear portion of the sperm is of real importance in the essential act of 

 fertilization ; the nutritive tail or cap simply dissolves away. After the sperm- 

 nucleus has penetrated to the center of the ovum, and after the extrusion of the 

 polar bodies is quite completed, we have to deal with two nuclei, not only 

 closely approximate in structure, but alike in further history-. 



In Carnoy's type, both male and female nuclei contain two chromatin 

 elements, in the form of bent rods ; and before union takes place, these 

 undergo a marked modification, the same in both cases. Round the chromatin 

 rods vacuoles are formed, limiting them from the surrounding protoplasm ; into 

 these the rods send out anastomozing processes, after the fashion of little 

 rhizopods ; gradually the rods thus resolve themselves into a network, in the 

 meshes of which minute " nucleoli " are also demonstrable. 



Fig. 45.— The Process of Fertilization. — After Boveri. — a, female pronucleus; b, polar bodies; c. male 

 nucleus; d, sperm-cap ; at; chromatin elements of uniting united female and male nucleus (a and c); 

 e, protoplasmic centers; /, 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. YI-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 



