With some important exceptions (Chang & Hancock, 1967), fertilization of mammalian eggs is species-specific in that the eggs of a species can be penetrated only by spermatozoa of the same species. The analysis of the species-specificity of fertilization is very difficult when in-vivo systems are used. In-vitro systems, on the other hand, eliminate many factors and enable us to examine directly the interactions between the eggs and spermatozoa.
Barros (1968) inseminated rat eggs in vitro with capacitated golden hamster spermatozoa and found that only one out of 294 eggs had been penetrated. In this egg, two spermatozoa had penetrated the zona pellucida but failed to enter the vitellus. On the other hand, none of 280 mouse eggs inseminated in vitro with capacitated hamster spermatozoa were found to be penetrated.