STRUCTURE AND FUNCTION IN MAMMALIAN EGGS 



45 



appearance, were found up to the 8-cell stage (Austin and Braden, 

 1953b). Piko and Bomscl-Helmreich (i960) have recorded triploid 

 and mosaic (3N/2N) embryos at mid-gestation (11 days) in rats at 



Fig. 31 

 Telophase first-cleavage spindle in a polyspermia rat egg. X 1,700. 

 (From Austin and Braden, 1953b.) 



a frequency corresponding to that of polyandry, but were unable 

 to find any at later stages. 



The other group of trinuclear eggs, namely those that have one 

 male and two female pronuclei, display the condition of polygyny 

 and can originate in three different ways : (a) The spermatozoon may 

 enter an egg deriving from a binuclear oocyte. Since binuclear 

 oocytes seem rarely to survive to maturation (p. 20), this source of 

 polygyny must be considered a most infrequent one. (b) The first 

 polar body may fail to form after the first meiotic division has gone 

 through to telophase; consequently, two second meiotic spindles 

 develop and lead to the presence of two female pronuclei in the 

 ootid. This also seems to be a most uncommon mechanism, but it 

 has been detected in untreated animals — in an outbred stock of 

 mice (V) at an incidence of about 2 per cent (Braden, 1957). (c) The 

 second polar body may fail to form after the second meiotic division 

 has gone through to telophase. This is probably the commonest of 

 the three processes responsible for the presence of two female 

 pronuclei and it has been induced under experimental conditions. 



