114 MAMMALIAN FERTILIZATION 



in vitro can be accomplished as a routine experimental proce- 

 dure." Thus in dealing with mammalian fertilization, the fertili- 

 zation reaction as elucidated in the marine species by Hartmann 

 (1949), Runnstrom (1952), and Tyler (1955) is completely ob- 

 scure. 



Activation and Parthenogenetic Development 

 of Mammalian Eggs 



In the eggs of mammals, as in those of marine species, the im- 

 mediate consequences of sperm penetration include the develop- 

 ment of a block to prevent polyspermy, the shrinkage of the 

 vitellus, and the resumption of the second maturation division 

 leading to the formation of the second polar body. These proc- 

 esses indicate the activation of the egg from a passive state to an 

 active one. However, activation of eggs even to parthenogenetic 

 development can be achieved by artificial means. Discussions of 

 parthenogenesis include the treatises of Rostand ( 1950 ) and 

 Beatty (1957) and brief reviews of the literature by Tyler 

 (1941), Thibault (1949), Chang and Pincus (1951), Smith 

 (1951), and Austin and Braden (1954b). 



The elevation of the fertilization membrane in the egg of lower 

 organisms after sperm penetration is one of the earliest criteria 

 of activation, but a fertilization membrane has not been described 

 in mammalian eggs except, perhaps, for one possible instance in 

 the hamster egg (Venable, 1946). Since the mammalian egg at 

 the time of fertilization is enclosed not only with a thick zona 

 pellucida but also with the corona radiata and cumulus oophorus, 

 the observation of any change of the vitelline membrane in a liv- 

 ing egg is extremely difficult. 



The shrinkage of the vitellus after activation either by sperm 

 penetration or by artificial means is not a reliable criterion. The 

 tonicity of suspending medium and the method as well as the 

 time interval of preparation of eggs for examination are possible 

 causes of shrinkage. In the vitellus of the rat egg, a 14% reduc- 

 tion in volume occurred in the fertilized tubal eggs ( Gilchrist and 

 Pincus, 1932). This is confirmed by Austin and Braden (1954b), 

 who reported also that shrinkage of the vitellus was not consist- 



