STRUCTURE AND FUNCTION IN MAMMALIAN EGGS 



95 



response in the hamster egg is illustrated in Fig. 73. Possibly, in 

 mammalian eggs that exhibit the zona reaction, but lack distinct 

 cortical granules, the active agent is carried in a more dispersed form 

 in the vitelline cortex. 



Fig. 73 



Diagrams of golden-hamster eggs to show the possible rela- 

 tion between sperm attachment, disappearance of cortical 

 grannies and spread of the zona reaction. 



The zona pellucida may be responsible in some instances for the 

 failure of heterologous fertilization. Viable hybrids are known in 

 a wide range of animals (Gray, 1954) and cross-insemination between 

 S Sylvilagus and + Oryctolagus (Chang and MacDonough, 1955; 

 Chang, i960), and S Lepus and £ Oryctolagus (Adams, 1957; R. G. 

 Edwards, personal communication, i960) was shown to result 

 in early embryos that pass through apparently normal cleavage, 

 though they degenerate soon afterwards. On the other hand, 

 persistent failure of sperm penetration has been reported after 

 artificial insemination of rats with bull, mouse, guinea-pig, rabbit 

 and Mastomys spermatozoa, of mice with rat, Apodemus, Microtus 

 and Mastomys spermatozoa, and of Mastomys with mouse and rat 

 spermatozoa — with the single exception of a Mastomys egg that was 



