28 THE MAMMALIAN EGG 



or else that nucleoli can become detached from their loci, leaving 

 them free to generate further nucleoli. Neither of these alternatives 

 is consistent with the generally accepted idea of the mechanism of 

 nucleolus formation. Total nucleolar volume is about 10 per cent 

 of the nuclear volume; by contrast, the proportion is only about 

 i per cent in most tissue-cell nuclei (Vincent, 1955). The male 

 pronucleus of the rat egg maintains a volume of about two-and-a- 

 half times that of the female pronucleus, and approximately the 

 same relationship holds also for number and total volume of 

 nucleoli (see also Blandau and Odor, 1950; Odor and Blandau, 

 1951b; Dalcq, 1955b). 



The pronuclei of other mammalian eggs have not been studied 

 in such detail as those of the rat egg, but certain similarities and 

 differences are evident. Mouse pronuclei tend generally to resemble 

 rat pronuclei, though they usually have fewer nucleoli and often 

 show a single nucleolus at presumed full development. In the 



wcw* 



Fig. 21 

 Rabbit pronuclei. X 1,500. 



mouse, as in the rat, the male pronucleus is much larger than the 

 female. A moderate pronuclear disparity is seen in the eggs of the 

 guinea-pig, rabbit (Fig. 21), multimammate rat, Chinese hamster 

 and Libyan jird (Fig. 22), but it is uncertain whether it is the male 



