BIOLOGY OF EGGS AND IMPLANTATION 



849 



TABLE 14.6 

 The fertilizable life of the mammalian ovum 



interference with either the normal proc- 

 esses of producing or liberating evocators, 

 or the capacity of the embryonic tissues to 

 respond to induction. Of special interest is 

 the finding that the older fertilized eggs 

 frequently gave rise to teratomatous pro- 

 liferations in the endoderm. When these 

 tumor-like masses were transplanted to 

 older larvae they grew rajjidly and metasta- 

 sized. Needham (1950) proposed that, be- 

 cause the primary evocator, the principal 

 sex-hormone, and various carcinogens be- 

 long to the steroid compounds, the effect of 

 over-ripeness may be related to a disturb- 

 ance of embryonic sterol metabolism. 



The fertilizable life of the mammalian 

 ovum has been experimentally determined 

 in only a few rodents, carnivores, and ungu- 

 lates (Table 14.6). In the ferret, for exam- 

 ple, Hammond and Walton (1934) found 

 that the ovum remains capable of fertiliza- 

 tion for not more than 30 hours after ovula- 

 tion. In the rabbit, delay in fertilization 



results in lowered fertility and smaller size 

 of litters (Hammond, 1934). In the hamster 

 50 per cent of ova are incapable of fertiliza- 

 tion 4 to 5 hours after ovvdation (Chang 

 and Fernandez-Cano, 19581. 



In rats the spermatozoa may penetrate 

 eggs which have been aged 12 hours before 

 fertilization or to a point of devitalization 

 but not of death. In such eggs they may 

 even undergo transformation into the male 

 pronuclei and form segmentation spindles, 

 but the female nucleus in the same egg 

 either fails to develoj) or fragments into a 

 number of nuclei of varying sizes. Even 

 though 70 per cent of the greatly over-ripe 

 rat eggs may be penetrated by spermatozoa, 

 various abnormalities of development result 

 which are not compatible with continued 

 growtii and development. Thus, at the time 

 of implantation only 4 per cent of the ex- 

 perimental rats are impregnated. Further- 

 more, the ova which do implant successfully 

 are retarded in their development, and the 



