

FERTILIZATION. 



29 



diately after entering, turn completely around so that the head points away 

 from the female pronucleus. An aster with its centrosomes next appears, 

 developing from, or in very close relation to the middle piece. The aster and 

 sperm nucleus now approach the female pronucleus, the aster leading and its 

 rays rapidly extending. On or before reaching the female pronucleus the 



a 



K 



^mm+-^ : 



FIG. 1 6. Fertilization of the eggs of the star-fish and sea-urchin. 



A, B, C, entrance of the sperm into the cytoplasm (star-fish). D, mature spermatozoon of the 

 sea-urchin; E-H, successive stages in the penetration of the sperm nucleus (c?AO and cen- 

 trosome (cf"C) into the cytoplasm; I-L, stages ; n the approach of the sperm nucleus (c?N) 

 to the egg nucleus (9./V), the division of the sperm centrosome (cfO) and the first 

 cleavage spindle. Fol, Wilson, from Conklin Heredity and Environment. 



aster divides into two daughter asters which separate with the formation of 

 the usual central spindle, while the two pronuclei unite in the equatorial 



