36 THE MAMMALIAN EGG 



hypodiploidy. The occurrence of subnuclei may be subject to 

 genetic influence: Braden (1957) found subnuclei far more com- 

 monly (7-2 per cent) in eggs undergoing fertilization in one colony 

 of mice (V stock) than in the others he investigated (o to 0-2 per cent). 



The frequency of occurrence of subnuclei in rat and mouse eggs 

 undergoing fertilization may be greatly increased by experimental 

 conditions, such as artificial insemination late in oestrus (Blandau, 

 1952), treatment of the eggs in situ with heat shock or systemically 

 administered colchicine (Austin and Braden, 1954b; Edwards, 1958a; 

 Piko and Bomsel-Helmreich, i960), or treatment of the spermatozoa 

 with ultra-violet or X-irradiation or with radiomimetic drugs 

 (Edwards, 1957^, b, 1958b) (Fig. 28b, c). 



Rudimentary parthenogenesis. The second-metaphase chromosome 

 group in unpenetrated eggs may not break up but instead give rise 

 directly to a single nucleus (Table 2) ; this would be diploid, unless 

 by a remote chance the first meiotic division has also failed, in 

 which case it would be tetraploid. In certain non-mammalian 

 animals, in which parthenogenesis occurs naturally or can be 

 induced artificially, a diploid nucleus is thus formed, the process 

 representing one of the mechanisms of 'regulation to diploidy' (see 

 also p. 76; and Tyler, 1941, and White, 1954). Alternatively, 

 unpenetrated eggs may show spontaneous resumption of the second 

 meiotic division and develop a single nucleus after the expulsion 

 of the second polar body (Table 2). This nucleus would be haploid 

 (or diploid if the first meiotic division had failed). Eggs of this kind 

 are rarely encountered in untreated subjects in mammals of most 

 species, but remarkably common in the golden hamster. In this 

 animal, about three-quarters of the eggs recovered some 20 hr after 

 ovulation were found to have undergone activation with expulsion 

 of the second polar body, and nearly one-third of them had devel- 

 oped single nuclei that resembled normal pronuclei (Austin, 1956a) 

 (Fig. 29). In this series of observations, only one normal-looking 

 2-cell egg was found at a later stage, so that the parthenogenesis of 

 the great majority of the hamster eggs must have been purely 

 rudimentary. Similar experiences were reported by Chang and 

 Fernandez-Cano (1958): among unpenetrated eggs recovered 13 to 

 40 hr after ovulation, about 40 per cent had formed single nuclei 

 with or without emission of the second polar body. Uninuclear eggs 

 have also been reported in untreated rats, mice and voles (Austin 

 and Braden, 1954c; Austin, 1957b), but it was not known whether 



