Studies on Marsupial Reproduction. 3. Normal and delayed pregnancy in Setonis Brachyurus.

Abstract

The period between copulation and birth in Setonix brachyurus Quoy and Gaimard is about 27 days. Approximately 1 day elapses between copulation and fertilization. The gestation period is slightly less than the length of one oestrous cycle. The tuba1 journey of the fertilized egg occupies only about 1 day. The unilaminar blastocyst 3-4 days after mating, with its enclosing membranes, measures about 0.3 mm diameter. At this stage the corpus luteum is not fully developed and a luteal phase is not evident in the uteri. Bilaminar blastocysts and later embryonic stages are associated with a well-developed corpus luteum and luteal changes in the uteri. Towards the end of pregnancy the corpus luteum shows degenerative changes and ovaries and uteri exhibit evidence of approaching oestrus. The allantois of the uterine embryo does not form a placental structure. A well-developed, vascular, trilaminar yolk-sac placenta occurs during the later stages of embryonic life. Oestrus and ovulation occur shortly after the female gives birth. The corpus luteum formed following post-partum ovulation remains of small size and functions during lactation of the young in the pouch as the corpus luteum of lactation. Luteal changes do not occur in the uterus during this time and a lactation anoestrus takes place. Removal of the young from the pouch causes growth of the corpus luteum of lactation and the onset of luteal changes in the uterus. If fertilization takes place at post-partum oestrus the resulting embryo remains as an unimplanted blastocyst of small size during the time the pouch is occupied by a suckling foetus. This quiescent embryonic stage will proceed to normal development following removal of the young from the pouch. Blastocysts may remain unimplanted for up to 5 months. The probable significance of the gestation period in relationship to the length of the oestrous cycle and the placental structure is discussed. The ovulation-inhibiting function of the corpus luteum of lactation and its later luteal function are compared. The mechanism of delayed pregnancy in Setonix is discussed and comparisons are made with known cases of delayed birth in other mammals. Some concluding remarks are made on the breeding habits of Setonix in relation to its environment.