STRUCTURE AND FUNCTION IN MAMMALIAN EGGS 75 



Odor, 1955). In mammals, the chromosomes in the first polar 

 body may remain clumped together, may undergo to varying 

 degrees a second meiotic division, or may become scattered in the 

 polar-body cytoplasm. Nucleus formation is most uncommon. On 

 the other hand, though chromosome scatter can also occur in the 

 second polar body, an interphase nucleus is frequently seen ; Braden 

 (1957) notes that in mice a nucleus is reconstituted in the second polar 

 body so often that its presence can serve to distinguish between 

 the two polar bodies. Consistently, Ward (1948) never saw nuclear 

 reconstitution in the first polar body in the hamster egg, though 

 it did occur in the second. 



Mammalian tubal eggs are often recovered with no polar bodies 

 (before sperm penetration) or only one polar body (during fertiliza- 

 tion) owing to the break-up of the first polar body; the frequency 

 of this occurrence varies widely with strain and species. In the 

 hamster (Austin, I956d) and field vole (Austin, 1957b), the first polar 

 body persisted in all the freshly ovulated eggs examined; in rabbits, 

 the incidence of persistence was 88 per cent (Austin and Bishop, 

 !957b)» whereas in the mouse it was 10 per cent (Sobotta, 1895), 

 and, in rats, only 2 per cent (Sobotta and Burckhard, 1910), 1*3 per 

 cent (Austin and Braden, 1954b) or 6 per cent (Odor, 1955). 



Emission of a polar body can suffer inhibition, either spontane- 

 ously or artificially, and this follows directly from failure of the 

 meiotic division to proceed beyond metaphase or anaphase, or to 

 failure of the telophase spindle to undergo rotation. Inhibition of 

 polar-body emission appears to be an inherited tendency (p. 45) and 

 to be favoured by delay in the time of fertilization (p. 46); emission 

 can be inhibited in rats by treatment with colchicine (p. 46). The 

 consequences of polar-body inhibition for pronuclear development 

 have already been discussed (p. 41 et seq. and Table 2) ; the genetic 

 consequences are dealt with systematically by Beatty (1957). 



In general, the larger the egg, the relatively smaller the polar 

 body, but this is not a strict relationship — rodent eggs tend to have 

 disproportionately large polar bodies (see, for example, the guinea- 

 pig egg in Fig. 38). In any one species, the size of the polar body is 

 normally fairly constant, but under some circumstances it can vary 

 greatly. Presumably, the determining factor is the position taken 

 up by the meiotic spindle relative to the egg surface; experiments 

 on the eggs of the gastropods Crepidula (Conklin, 1917) an d 

 Ilyamssa (Clement, 1935) showed that displacement of the meiotic 



