FERTILIZATION AND CLEAVAGE 81 



actual penetration and pronucleus formation occurred in his 

 attempts to fertilize rabbit eggs with rat sperm. No figures 

 showing actual sperm penetration are given in this paper. 

 The nuclear configurations shown may, in fact, occur in 



Fig. 26. Rabbit ovum from sterile mat- 

 ing treated with trypsin solution. Note 

 many "polar" bodies. See text. 



ova cultured in vitro with no sperm added (see Chapter 

 VIII). 



The inamediate effect of semination (and fertilization) 

 upon mammalian ova is a definite shrinkage of the vitellus 

 (Pincus and Enzmann, 1932). Quantitative estimates of this 

 shrinkage in rat eggs have been made by Gilchrist and 

 Pincus (1932). In Table XII are presented their data on 

 folhcular and tubal ova. They show that a 14 per cent reduc- 

 tion in volume occurs in fertilized tubal ova. Furthermore, 

 when unfertilized ova are exposed to sperm suspensions 

 a similar shrinkage occurs (Table XIII). This shrink- 

 age is not due to polar body extrusion since it occurs in vitro 

 within 5 to 10 minutes, and polar bodies are normally ex- 

 truded at 45 minutes to 1 hour after semination in vitro 

 (Long, 1912). The ova apparently increase somewhat in 

 volume after this initial shrinkage. Krasovskaja (1935a) 



