46 THE MAMMALIAN EGG 



The application of heat to the Fallopian tubes of mice 3 hr after 

 mating increased the incidence of second-polar-body suppression 

 from 0-5 to 12-4 per cent (Braden and Austin, 1954b). Studies on 

 special groups of mice have revealed that, in the outbred stock just 

 mentioned (V), suppression of the second polar body occurs at 

 higher incidence than that of the first, namely, between 4 and 5 per 

 cent (Braden, 1957). Polar-body suppression is evidently a geneti- 

 cally controlled factor in these animals, and is the probable cause 

 of the triploidy recognized to be relatively common in this strain 

 of mice (Beatty and Fischberg, 1951). In contrast to the effect of 

 delayed mating in the rat, which often increases the frequency of 

 polyandry as already noted, delayed mating in the hamster has been 

 found to produce an even more striking increase in polygyny, thirty 

 out of eighty-eight penetrated eggs (34 per cent) showing this con- 

 dition (Chang and Fernandez-Cano, 1958). Polyandry was not 

 increased in incidence. Recent observations on pig eggs reveal that 

 the frequency with which polygynic eggs are found is greatly 

 increased, from o to 21 per cent, if coitus or artificial insemination 

 is effected more than 36 hr after the onset of oestrus (Thibault, 

 1959). Intraperitoneal injections of colchicine have been reported 

 to cause second-polar-body suppression at a high incidence (38 per 

 cent of penetrated eggs) in rats, if given 2 hr after mating; injection 

 at 2 \ hr resulted in suppression in only 11 per cent of eggs (Piko and 

 Bomsel-Helmreich, i960). (See also Fischberg and Beatty, I95 2 -) 



Suppression of the second polar body can accompany polyspermy 

 and so give a quadrinuclear egg containing two female and two 

 male pronuclei, and this has been reported in a pig egg (Thibault, 

 1959) and a rat egg (Austin and Walton, i960). Alternatively, an 

 egg may complete maturation normally but be entered by three 

 spermatozoa (trispermy) and so come to have one female and three 

 male pronuclei. The occurrence has been reported in untreated 

 rats (Austin, 1951b; Austin and Braden, 1953b), and in animals in 

 which hyperthermia had been induced (Austin, 1956b). Although 

 no measurements are recorded of the nuclei in trispermic eggs, it is 

 clear from the general appearance that the female and all the male 

 pronuclei each attain a smaller size than that of the corresponding 

 pronuclei in normal eggs (Fig. 3 of). One example of spontaneous 

 tetraspermy has been described in a rat egg — the five nuclei were all 

 well formed, the four male pronuclei being equally larger than the 

 female pronucleus (Piko, 1958). Tetra- and pentaspermic eggs have 



