OF THE CELL THEORY 185 



made in the years 1883 and 1884, rank deservedly 

 among the most remarkable of microscopical 

 triumphs. Van Beneden finds that after extrusion 

 of the polar bodies, and entrance of the sperma- 

 tozoon into the egg, the two pronuclei, male and 

 female, which are precisely equal in all respects, 

 come very close together but do not fuse directly. 

 Each pronucleus bears at first a single much convo- 

 luted and varicose thread of chromatin, which soon 

 divides transversely into two, each of which 

 becomes bent into a U-shaped loop. There are 

 then four loops in all, two male and two female. 

 Each loop now splits longitudinally into two sister 

 threads. A spindle figure with pole-bodies and 

 polar rays at its apices now appears, and the out- 

 lines of the pronuclei, previously distinct, disappear. 

 The chromatin loops, of which owing to the longi- 

 tudinal splitting there are now eight, four male 

 and four female, take up a position at the equator 

 of the spindle. The two sister threads of each 

 pair now separate, one moving towards one pole 

 of the spindle the other towards the opposite pole; 

 so that at each pole of the spindle there is a group 

 consisting of two male threads and two female 

 threads. Each group now forms a daughter 

 nucleus, and then the entire egg divides into the 

 first two segmentation cells. This account agrees 

 with Nussbaum's as regards the equal division of 

 male and female threads between the first two 

 segmentation cells ; but differs inasmuch as 

 according to Nussbaum, the two pronuclei fuse 

 directly, while, according to van Beneden, there is 



