STRUCTURE AND FUNCTION IN MAMMALIAN EGGS 23 



of these groups has a haploid number of chromosomes. In many 

 non-mammalian animals, the first polar body passes through a 

 division equivalent to the second meiotic division of the oocyte ; 

 thus, four haploid cells can be formed, one ootid and three polar 

 bodies. This is analogous to the formation of four haploid sperma- 

 tids from each primary spermatocyte in the course of spermato- 

 genesis. With both systems, the final cells each have a genotype 

 that can be considered unique, because the pattern of chiasma 

 formation is not fixed and each homologous chromosome pair may 

 form from one to ten chiasmata — under these circumstances the 

 number of possible genie recombinations is very large indeed 

 (White, 1954). This means that the hereditary characters contributed 

 by the female will vary in detail with each egg. Beatty (1956b), 

 Beatty and Napier (i960), Beatty and Sharma (i960) and Sharma 

 (i960) have produced evidence that the genotype of the spermato- 

 zoon influences its phenotype, and so the possibility presents itself 

 that variations in the genotype of eggs might also be recognizable 

 from their visible features. To some extent this has been found to 

 be so: the eggs of some inbred strains of mice can be distinguished 

 from those of other strains by the appearance of the cytoplasm 

 (Braden, 1959, 196 1). (An excellent discussion on the genetic 

 individuality of spermatozoa is given by Bishop, i960.) 



As a spontaneous anomaly or through experimental treatment, 

 either of the meiotic divisions may be inhibited (see Beatty, 1957). 

 If the first anaphase separation is blocked, the chromosomes remain 

 together, still constituting a tetraploid group; when the second 

 division takes place and the chromatids separate, two diploid 

 chromosome groups are formed, one passing into the polar body 

 and the other remaining within the vitellus. The fertilization of 

 such an egg gives a triploid embryo. If the first meiotic division is 

 inhibited after anaphase separation of the chromosomes, it is 

 possible that two second maturation spindles will develop; the 

 presence of two such spindles, occasionally reported in the literature 

 (Pesonen, 1946a, b; Vara and Pesonen, 1947; Braden and Austin, 

 1954b; Austin and Bishop, 1957b; Braden, 1957), can therefore be 

 ascribed not only to the maturation of a binucleate oocyte but also 

 to the form of inhibition just referred to. The second meiotic 

 division may likewise be blocked at either of two points; the 

 outcome in this case could be the development of a single diploid 

 female pronucleus or of two haploid ones, both conditions possibly 



