STRUCTURE AND FUNCTION IN MAMMALIAN EGGS 39 



Sometimes, when the second mciotic division proceeds spon- 

 taneously or after artificial activation, in impenetrated eggs, it is not 

 succeeded by the expulsion of the second polar body and, conse- 

 quently, two (haploid) nuclei are formed. This is a rarer event than 

 the formation of a single nucleus but has been reported in the rabbit 

 (Thibault, 1949), rat (Austin and Braden, i954d), mouse (Braden and 

 Austin, 1954c) and hamster (Austin, 1956a; Chang and Fernandez- 

 Cano, 1958). The two nuclei can look remarkably like normal 

 male and female pronuclei, but are considered incapable of under- 

 going normal syngamy, at least in the rabbit egg, and to be unlikely 

 therefore to lead to any further development (Thibault, 1949). 



Gynogenesis and androgenesis. The presence of a single nucleus in 

 an egg that has been penetrated by a spermatozoon is generally 

 owing to failure of either the male pronucleus, as in gynogenetic 

 development, or of the female pronucleus, as in androgenetic 

 development. (It is just possible that the uninuclear state can 

 arise from fusion of male and female, or two male, pronuclei: 

 Pesonen, 1949; Austin and Braden, 1954b.) The nuclei are haploid 

 unless one or other of the meiotic divisions has been inhibited, but 

 they are nevertheless capable of growing in an apparently normal 

 way to a large size, sometimes becoming bigger than a normal 

 pronucleus. Instances of uninuclear eggs possibly representing 

 spontaneous early gynogenesis and androgenesis have been described 

 in rats (Austin and Braden, 1954c), mice (Austin and Bruce, 1956) 

 and hamsters (Austin, i956d), and after heat treatment in rats (Austin 

 and Braden, 1954b). Attempts to induce gynogenesis artificially in 

 the mouse by X-irradiation of the testes of the males yielded thirteen 

 uninuclear eggs that could have been undergoing this form of 

 development (Bruce and Austin, 1956), but evidence indicated that 

 normal cleavage was most unlikely to have ensued. X-irradiation 

 or ultra-violet irradiation of the spermatozoa, or injection of 

 colchicine solutions into the uterus through the cervix, resulted in 

 the production in mice of some instance of early gynogenesis and 

 androgenesis, and there were indications that, while neither form of 

 development was likely to be protracted, the androgenetic embryo 

 was a little the more viable (Edwards, 1954, 1957a, b, 1958b). 

 Intraperitoneal injections of colchicine in rats, given 2 J hr after 

 mating, have resulted in a high incidence of androgenetic eggs 

 (3 8 per cent of penetrated eggs) ; the time was highly critical : with 

 similar injections given at 2 hr after mating, the incidence was 



