40 



THE MAMMALIAN EGG 



only o*9 per cent (Piko and Bomsel-Hclmreich, i960). The mechan- 

 ism involved appeared to be the exclusion of the whole of the 

 female chromatin in a polar-body-like structure, formed amito- 

 tically. 



Aiieuganty. Anomalies of pronuclei may involve, not the number 

 of male or female pronuclei present in an egg, but the ploidy of 

 one or both of the pronuclei (Table 2). Aneuploidy in pronuclei 

 constitutes the state of aneugamy. The condition can arise through 



TABLE 2 



The nine theoretically possible kinds of Ootid, with respect to 

 Number and Ploidy of Polar Bodies (pb) and Female Pronuclei (pn), 



THAT COULD ARISE THROUGH SUPPRESSION OF ONE OR BOTH POLAR BODIES. 



In the absence of sperm penetration, these classes describe forms of partheno- 

 genetic eggs. If fertilization is initiated, the corresponding ootids would display : 

 (a) normal fertilization; (b), (d) and (e) aneugamy ; (c), (f), (g), (h) and (i) polygyny. 



fertilization by a normal spermatozoon of an egg deriving from a 

 uninuclear octaploid primary oocyte (8N in dna content and 

 chromatid count, 4N in chromosome number) or of an egg in 

 which one or both mciotic divisions have failed, or through fertiliza- 

 tion by a polyploid spermatozoon. Clearly, the number of com- 

 binations of these variables is large, so that a wide variety of forms 

 of aneugamy are possible. This group of anomalies is, however, 

 likely to remain largely hypothetical until studies are made on the 

 chromosome complements of pronuclei, which will probably be 



