STRUCTURE AND FUNCTION IN MAMMALIAN EGGS 99 



are much more constant among different species than are those of 

 the zona pellucida. 



The cumulus masses surrounding freshly ovulated rodent eggs arc 

 quickly broken up, and the eggs thus denuded, by treatment with 

 sperm suspension (as noted by Schenk, 1878) or with solutions of 

 hyaluronidase. This is not true, however, for oocytes recovered 

 from large ovarian follicles, and the difference is probably to be 

 attributed to the firmer attachment between the follicle cells before 

 ovulation. Hyaluronidase solutions also fail fully to denude ovu- 

 lated rabbit, dog and cat eggs; the more densely-packed cells in the 

 immediate vicinity of the egg, the corona radiata, evidently retain 

 sufficient direct attachment to the egg and to each other to maintain 

 their positions in the absence of matrix. 



In studies with the rodents and the rabbit, it has frequently been 

 remarked that the cumulus disintegrates more rapidly in mated 

 animals than in those that have not mated. It is reasonable to hold 

 that disintegration is owing to the action of hyaluronidase liberated 

 from spermatozoa that reach the site of fertilization. The hyaluroni- 

 dase carried by spermatozoa is probably associated with the acrosome 

 (Leuchtenberger and Schrader, 1950; Schrader and Leuchtenbergcr, 

 1951; Bishop and Austin, 1957), and in ejaculated and epididymal 

 spermatozoa appears to be released only by the moribund cells (see 

 Mann, 1954), in which the acrosome becomes visibly changed or 

 detached (Austin and Bishop, 1958b). Before spermatozoa can take 

 part in fertilization, they need to undergo a form of physiological 

 preparation called 'capacitation' in the female genital tract (Chang, 

 1951a, 1955b, 1958; Austin, 1951a, 1952b; Noyes, 1953; Austin and 

 Braden, 1954a; Noyes, Walton and Adams, 1958); this evidently 

 involves a change in the acrosome of the living spermatozoon 

 resembling in appearance that shown by the acrosome of the 

 moribund spermatozoon (Austin and Bishop, 1958c). When tested 

 under specific conditions /'// vitro, epididymal and ejaculated sperma- 

 tozoa are unable to pass into the cumulus matrix, whereas cumulus 

 masses recovered from mated animals are often found to contain 

 spermatozoa that move freely through the cumulus — these sperma- 

 tozoa exhibit the acrosome change. It is therefore inferred that the 

 acrosome alteration involved in capacitation permits the release of 

 hyaluronidase, which enables the spermatozoon to penetrate the 

 cumulus by liquifying the matrix in the vicinity of its head (Austin, 

 1948, 1960c, i96id). The altered acrosome is easily detached and 



