STRUCTURE AND FUNCTION IN MAMMALIAN EGGS 25 



when they come into contact with each other. By the time nucleoli 

 are evident, a distinct nuclear membrane can be seen. In the deriva- 

 tion of the female pronucleus, nucleoli appear in the irregular mass 

 of aggregated chromosomes, and an encircling nuclear membrane 

 soon makes its appearance. 



C 



Fig. 17 

 Transformation of the rat sperm head into a male pronucleus. (Drawn 

 from photographs taken of the changes as they proceeded in vitro.) 



The pronuclei grow rapidly, and this involves not only increase 

 in nuclear volume and total nucleolar volume, but also increase, at 

 least at certain stages, in the number of nucleoli (Austin, 1952a). In 

 the living rat egg examined by phase-contrast microscopy, the new 

 nucleoli appear to be generated at the nuclear membrane, often 

 seeming at such times to indent the membrane quite distinctly. These 

 nucleoli are themselves distorted from the spherical, and the whole 

 effect suggests that the nucleoli have surface tension and are capable 



