STRUCTURE AND FUNCTION IN MAMMALIAN EGGS 43 



normal incidence of the condition among penetrated eggs varies 

 somewhat in different species but has been found generally to be of 

 the order of i or 2 per cent (Table 3) but in the pig it may be as 

 high as 10 per cent (Pitkjanen, 1955). Polyandry may become much 

 more common with coitus late in oestrus, and following heat treat- 

 ment (Table 3). Piko and Bomsel-Helmreich (i960) found that 

 hyperthermia induced in rats produced 8 to 10 per cent polyspermic 

 (dispermic) eggs in the Sherman and Long-Evans strains, but only 

 3-5 per cent in the Wistar CF strain. Hancock (1959, 1961) reported 

 that the incidence of trinuclear eggs in pigs allowed coitus at the 

 start of oestrus or at 24, 30 and 40 to 48 hr later was o, 3, 13 and 

 41 per cent, respectively. His cytological evidence indicated that the 

 trinuclear state could be ascribed chiefly to polyandry. Thibault 

 (1959), on the other hand, maintained that the principle effect of 

 late mating or insemination in the pig is an increase in the incidence 

 of polygyny, the increase for polyandry being relatively small (from 

 1-8 to about 12 per cent). 



The general uncommonness of polyandry under normal circum- 

 stances is attributable chiefly to the relatively small number of 

 spermatozoa reaching the site of fertilization (see Braden and Austin, 

 1954a) and to the fact that either the vitelline surface or the zona 

 pellucida, or both, tend to become impermeable to spermatozoa 

 after the entry of the first (see pp. 88 and 92). 



Polyandry has been studied in some detail in the rat. It was 

 observed that the two male pronuclei develop in remarkably close 

 parallel with each other (Fig. 30a, b, c and e), a feature that may be 

 owing to the operation of a co-ordinating influence (see p. 47) or 

 to the necessarily closely synchronous entry of the spermatozoa. 

 The volumes achieved by the pronuclei at full development were 

 individually always less than those of the corresponding normal 

 pronuclei, and this was true too for nucleolar volumes (Fig. 3od, e 

 and f). Indeed, the sum of the nuclear volumes (about 7,300 tt 3 ) and 

 of nucleolar volumes (about 800 /x 3 ) in polyandric eggs did not 

 differ significantly from the corresponding figures for normal eggs 

 (about 8,000 /x 3 and 800 /x 3 , respectively). At the approach of 

 syngamy, contact occurred just as often between the two male 

 pronuclei as between a male and the female, testifying to a lack of 

 specificity in the forces that draw the pronuclei together at this phase 

 of fertilization. By all appearances, the general course of syngamy 

 in polyandric eggs was the same as in normal eggs, except for the 



