The first barrier presented to rabbit spermatozoa at the site of fertilization is the cell mass which comprises the cumulus oophorus and, immediately surrounding the ovum, the corona radiata. Fertilization in the mouse, rat and rabbit occurs before dispersion of these granulosa cells (Lewis & Wright, 1935; Austin, 1948a, b; Chang, 1951a). In studies of the interaction of spermatozoa with the cumulus of the rat, rabbit and hamster in vitro, Austin ( 1960) observed that epididymal or ejaculate spermatozoa were apparently unable to penetrate the cumulus oophorus, whereas under similar conditions spermatozoa were seen moving freely through the cumuli obtained from mated animals. This observation led Austin to suggest that capacitation, which occurs while the spermatozoa are in the female tract (Austin, 1951; Chang, 1951b), may facilitate sperm passage through the cumulus oophorus to the surface of the zona pellucida. This suggestion has been tested in the rabbit, in vivo, in the manner described below.
Spermatozoa flushed from rabbit uteri with Hanks' solution 12 to 14 hr after natural mating were concentrated by centrifugation at 1000 rev/min for 5 min.