. 



*** 



98 THE MAMMALIAN EGG 



liberating denuded eggs. The mechanism responsible for this dis- 

 integration is unknown, though evidence shows that enzymic acti- 

 vity or mechanical movement within the Fallopian tube is partly 

 responsible, at least in the rabbit (Swyer, 1947). In the rat, mouse 



and hamster, it seems possible 

 that the cells in the cumulus 

 surrounding freshly-ovulated 

 eggs are still too tightly packed 

 to permit sperm penetration 

 into the eggs : penetration was 

 found to begin 3 to 4 hr after 

 ovulation, whereas in the rab- 

 bit it appears to start imme- 

 diately after ovulation (Austin 

 and Braden, 1954a; Austin, 

 I956d; Strauss, 1956). Braden 

 (1958b) showed that in two 

 inbred strains of mice the 

 delay in sperm penetration 

 FlG - 75 differed in duration and so also 



radiata^xIsO. Ulbal ^^ ^ ^ C ° r ° Ua did the dellsit y ° f the CUmU " 



lus and the rate at which the 

 investment ultimately broke up. Study of the heritability of these 

 features confirmed the belief that they are determined by the geno- 

 type of the female. It has also been shown that the density of the 

 cumulus can be reduced, and the delay in sperm penetration short- 

 ened, by treating the females with injections of gonadotrophs 

 which provoke ovulation (Braden, i960). 



The matrix of the cumulus contains protein but is largely com- 

 posed of the acid mucopolysaccharide known as hyaluronic acid, 

 which is also a constituent of several tissues, notably synovial fluid, 

 umbilical cord, vitreous humor, aqueous humor and the ground 

 substance of connective tissue. It is readily liquefied by proteolytic 

 enzymes, such as trypsin, chymotrypsin, pepsin and mould protease 

 (Braden, 1952, 1955), and by the specific enzyme hyaluronidase, 

 which spermatozoa carry. The permeability of the matrix to 

 solutes is perhaps slightly less than that of the zona pellucida, but 

 still sufficient to allow passage of substances of m.w. 1,200 (Austin 

 and Lovelock, 1958). The various properties of the cumulus matrix 



