REPROD UCTION. 



905 



it having come, how- 

 ever, from the male cell 

 and one-half from the 

 female cell. On the 

 commonly accepted 

 theory that this is the 

 hereditary substance, 

 the first segmentation 

 nucleus contains within 

 itself potentially all the 

 inherited qualities of 

 the future individual. 



While the head of 

 the spermatozoon is 

 making its way through 

 the substance of the egg 

 there appears beside it 

 a minute cytoplasmic 



FIG. 310. Stages in the fertilization of the egg (after Wilson). The 

 drawings were made from sections of the eggs of the sea-urchin, Toxo- 

 pneustes variegatus, Ag. 



A. The surface of the egg has become elevated to form c, the entrance- 

 cone for the spermatozoon ; the head (h) and the middle-piece (TO) of the 

 latter have entered the egg. 



B. Five minutes after entrance of the spermatozoon. The head (), 

 now the male pronucleus, has rotated 180 degrees, and has travelled 

 deeper into the ovum. The cytoplasm of the latter has become arranged 

 in a radiate manner about the middle-piece of the spermatozoon, now the 

 centrosome, to form the sperm-aster; e, the egg-nucleus, now the fe- 

 male pronucleus, is approaching the sperm-nucleus ; its chromatin forms 

 an irregular reticulum ; c, the entrance cone. 



FIG. 311. Stages in the fertilization of the egg (continued from Fig. 310). 



c. Ten minutes after entrance of the spermatozoon. The male and the female pronuclei have met 

 near the centre of the egg and the fusion has begun ; the former has become enlarged and its chromatin 

 has become loosely reticulated. The sperm-aster has become enormously enlarged. The single centro- 

 some has been divided into two, which lie upon either side of the sperm-nucleus. 



D. The pause thirty minutes after entrance of the spermatozoon. The two pronuclei have fused com- 

 pletely to form the first segmentation-nucleus, all trace of a distinction between paternal and maternal 

 chromatin being lost. The sperm-aster has become divided into two asters, which have moved to oppo- 

 site poles of the nucleus ; the astral rays have become shortened. The egg is now ready to .undergo 

 segmentation. 



