840 



SPERM, OVA, AND PREGNANCY 



Fig. 14.13. Further transformation of the sperm head into the male pronucleus, 1 and 2. 

 Note the large nucleolus, formed by fusion of smaller nucleoli. NC, nucleolus. This stage has 

 been reached approximately 5 hours after that in part 1, Figure 14.12, 1 and 2 (Austin, C. R., 

 1952). Developing male ( i ) and female ( $ ) pronuclei, in situ, as observed in living rat eggs, 

 3, 4 and 5. The entire sperm flagellum enters the ooplasm at the time of sperm penetration, 3 

 (Odor and Blandau, 1951). 



and number, certain of them coalesce, even- 

 tually producing one or two large nucleoli. 

 As the pronucleus grows, the optical density 

 of its nucleoplasm decreases to such an ex- 

 tent that it becomes clear and translucent. 



Although there may be considerable varia- 

 tion in the development of the pronuclei be- 

 tween the 9th and 19th hours after sperm 

 penetration, this is the time of active growth 

 of the pronuclei and of increase in the num- 



