Induction of oestrus, recovery of gametes, and the timing of fertilization events in the opossum, Didelphis virginiana.

Of 14 lactating opossums maintained in laboratory conditions, 13 mated 4.7-8.5 days after removal of pouch young. The time between this removal and onset of receptive oestrus was negatively correlated with the age of the pouch young. Mating generally occurred between 24:00 and 06:00 h, with ovulation following between 13:00 and 16:00 h. Each animal ovulated a mean of 29.6 eggs (range 19-40), approximately equal numbers coming from both ovaries. Spermatozoa were absent from the uterus and were present only in the oviducts during the periovular period. Those not cleared by flushing (1-160 x 103/oviduct) remained incarcerated in isthmic crypts lined by a simple cuboidal epithelium. Spermatozoa in crypts were paired, separating or single. The progressively motile cells flushed from the oviduct presented a similar pattern to that in the crypts, about 30% of spermatozoa were firmly paired, the others either loosely associated or single. Only single spermatozoa attached to ova. Monospermic fertilization followed shortly after ovulation, and no supplementary spermatozoa were present in the perivitelline space. Deposition of the mucoid layer on the zona pellucida then began, often before incorporation of the fertilizing spermatozoon by the vitellus was complete. The oviducal epithelium was formed throughout by ciliated and secretory cells. In the ampulla and upper isthmus, the secretory cells produced the mucoid material which formed a thick coat over the egg surface. Ovum transit through the oviduct was rapid, in one animal eggs had reached the uterus and acquired a shell within 15-20 h of ovulation.