24 THE MAMMALIAN EGG 



leading to a triploid embryo. Eggs with single female pronuclei 

 that could have been diploid were recovered from rats after colchi- 

 cine treatment (Austin and Braden, 1954b). The presence of two 

 female pronuclei may clearly come about through any of three 

 mechanisms : maturation of a binuclcar oocyte, or blockage at the 

 appropriate point of either the first or the second meiotic divisions. 

 Further consideration of the consequences of inhibition of meiotic 

 divisions is given particularly by Beatty (1951a, 1957), and also by 

 Austin (1960b), in Table 2 and on p. 40. 



The effect of sperm entry upon the egg, the first evidence of 

 which is the resumption of the second meiotic division and the 

 emission of the second polar body, is known as activation ; other 

 changes associated with this process are a reduction in vitelline 

 volume and a rearrangement of the cytoplasmic granules. If, on the 

 other hand, sperm penetration does not take place, the second 

 meiotic division may eventually be resumed spontaneously, marking 

 the beginning of parthenogenetic development — this is particularly 

 liable to happen in the golden hamster (Austin, 1956a; Chang and 

 Fernandez-Cano, 1958). In rats, mice and rabbits, the chromosome 

 group generally breaks up, chromosomes scatter through the cyto- 

 plasm and apparently later lead to the development of subnuclei. 

 The initiation of parthenogenesis may be achieved much more 

 commonly in these animals' eggs if they are subjected to certain 

 artificial stimuli (see p. 38). 



Pronuclear Growth and Development 



Two pronuclei take part in the normal process of fertilization, 

 the male pronucleus originating from the nucleus of the sperm head, 

 and the female pronucleus from the group of chromosomes that 

 remain within the vitellus after the expulsion of the second polar 

 body. The sperm-head nucleus consists principally of deoxyribo- 

 nucleoprotein which appears to be disposed in a compact state 

 resembling that of a crystal lattice (see Bishop and Walton, i960); 

 the chromosomes must presumably be there in a form appropriate 

 to the preservation of gene relations, but they are difficult to recog- 

 nize. The transformation of the sperm-head nucleus into a male 

 pronucleus involves loss of the characteristic shape, increase in 

 volume, apparently by a form of hydration, and a change in state 

 of the ground substance from solid to fluid (Fig. 17). At an early 

 stage, minute nucleoli make their appearance and grow, coalescing 



