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Ovariectomy during gestation in mammals usually results in abortion or death of the embryos. In some mammals, however, the embryos survive and can develop for a short time.

In ferrets, implantation occurs when ovariectomy is performed on Day 10 post coitum (p.c.) but resorption follows immediately (Buchanan, 1969). In the guinea-pig, implantation occurs if ovariectomy is performed more than 48 hr p.c. (Deanesly, 1960, 1966) but development continues normally only up to the 14th day. Ovariectomy of the nine-banded armadillo during delay results in precocious implantation (Buchanan, Enders & Talmage, 1956) which occurs 18 to 20 days after operation (Enders, 1966); embryonic and fetal development proceed normally (Enders & Buchanan, 1959). Marsupials, like armadillos, can dispense with their ovaries for the greater part of gestation. In the quokka, Setonix brachyurus, and the tammar, Macropus eugenii, the blastocysts

Australia is home to a unique assembly of mammals – the marsupials and monotremes. Despite this uniqueness, they have been largely ignored by the biomedical scientific community, and yet study of marsupials has contributed to modern research on reproduction, development, evolution, conservation, molecular and comparative genomics. My lifetime passion for these long-neglected Australian fauna has led to unexpected discoveries and insights that challenged assumptions and opened up new areas of international research. I used a range of disciplinary expertise to pursue the study of these unique mammals. My main experimental species has been the tammar wallaby that I have used as a model species to investigate and understand not only biomedical problems but also to provide knowledge that is critical for the continued conservation and management of Australia’s dwindling native mammals. This model provided more than a few surprises for me and my wonderful team of students, post-docs and collaborators about how hormones, genes and signalling molecules control reproductive biology and development in a wider context as well as how the interactions of the environment with mother and conceptus, with mother and fetus and mother and young ultimately control most aspects of successful reproduction in mammals.

Summary.

Blood sera and yolk-sac fluids were collected from pregnant tammar wallabies and analysed for free amino acids, proteins and glucose.

The total and individual concentrations of free amino acids at all stages of pregnancy were higher in yolk-sac fluids than in sera, with the exception of glutamic acid, which was at a higher concentration in serum.

Protein concentrations were lower in yolk-sac fluids than in the serum but the number of protein components in yolk-sac fluid, as determined by acrylamide gel electrophoresis, increased after implantation.

Glucose concentration in yolk-sac fluids also increased with the age of the embryo. These values were lower than those in sera before implantation, but higher than in sera after implantation.

These results indicate that the transfer and accumulation of nutrient materials in the yolk-sac is not only by simple diffusion from maternal serum, but also by selective transfer through the yolk-sac placenta.

Department of Zoology, University of Tennessee, Knoxville, Tennessee 37916, U.S.A.

(Received 8th July 1974)

The North American opossum, Didelphis marsupialis virginiana, is a polyovular, polyoestrous marsupial. The gestation period of 13 days is the same length as the luteal phase of the cycle (Hartman, 1923, 1925); the next pro-oestrous phase and ovulation are suppressed by the presence of sucking young in the pouch. During the breeding season, removal or loss of the young results in resumption of follicular growth and oestrous behaviour is evident 3 to 7 days later (Hartman, 1923; Renfree, 1974). After ovulation, the two uteri increase greatly in size and turgidity, due to an accumulation of fluid in the hypertrophied mucosa and the development of a rich supply of capillaries and venules (Harman, 1923). Ciliated surface cells are involved in the deposition of much ground substance in the stroma, and the secretory cells of the glands have

Summary. Oestradiol-17β concentrations were measured by radioimmunoassay in peripheral blood samples from 10 tammar wallabies after their pouch young were removed to terminate embryonic diapause. Oestradiol concentrations rose from 8·3 ± 1·2 pg/ml on Days 3 and 4 to a peak of 15·8 ± 2·9 pg/ml on Day 5, coincident with an increase in 'progesterone' concentrations, and then fell to 10·5 ± 2·7 pg/ml on Day 7. No changes in oestradiol concentrations were associated with parturition. Five females came into oestrus and mated 9·8 ± 6·1 h post partum; peak concentrations of plasma oestradiol (20·9 ± 2·1 pg/ml) occurred around the time of mating. None of the females that did not mate up to the end of the experiment at Day 30 had a rise in plasma oestradiol concentrations. Corpora lutea contained 20-100 pg oestradiol during pregnancy. The highest ovarian oestradiol content (> 1200 pg) was measured in whole ovaries containing Graafian follicles from full-term pregnant females. The rise in oestradiol concentrations at Day 5 may be important in the termination of diapause. The post-partum increase in plasma oestradiol concentrations coincides with oestrus. The source of this oestrogen appears to be the preovulatory follicle.

The placenta of the tammar wallaby, like that of most marsupials, is formed by close apposition of the yolk-sac membrane to the uterine epithelium. The yolk-sac fluid is rich in a variety of substances such as amino acids, glucose and protein which change quantitatively and qualitatively throughout gestation, the most marked changes occurring after attachment of the membrane to the uterus (Renfree, 1970).

Since many of the proteins resemble maternal proteins, they may have been transferred unchanged after attachment; conversely, they may have been synthesized by the embryo and the similarities may be due to the genetic similarities of the embryo and its mother. We have examined these two possibilities in the β-globulin proteins of yolk-sac fluid. The transferrins in this moiety of serum can be detected by virtue of their capacity to

Summary. Removal of the corpus luteum of pregnancy of the tammar wallaby interfered with successful parturition if carried out before Day 17 of the 27-day pregnancy. After removal at Days 17 and 21, 40% of animals gave birth but pouch young died within 24 h; if performed at Days 23 or 25, pouch young survived. However, surgery also affected sham-operated controls if performed between Days 15 and 21. In control animals, sodium pentobarbitone followed by halothane anaesthesia was the least disruptive anaesthetic procedure. This study shows that the corpus luteum has an essential role in parturition and subsequent survival of the neonate in the pouch.

M.R.C. Unit of Reproductive Biology, 2 Forrest Road, Edinburgh, and Department of Animal Genetics, University of Edinburgh, Edinburgh

(Received 25th September 1974)

The marmosets (F. Callithricidae), in common with other South American primates, do not menstruate. This is a major disadvantage in animals which otherwise have many attributes as experimental models for studies in reproductive physiology. The conventional methods of determining the stage of the ovarian cycle by means of vaginal smear cytology do not work for the common marmoset (Callithrix jacchus). The presence of spermatozoa in the vaginal smear is no indication of oestrus because marmosets mate frequently and apparently indiscriminately throughout the cycle and early pregnancy (Hearn & Lunn, 1974). The patterns of LH, oestradiol and progesterone in the peripheral plasma of the common marmoset show that this species has a clearly defined ovarian cycle of 16·4± 1·7 days (Hearn & Lunn, 1974), and it seemed possible that

Summary. Tammar wallabies were treated with progesterone injections or implants during late pregnancy to determine whether progesterone withdrawal was essential for parturition. Neither physiological (implanted group) nor pharmacological (injected group) levels of circulating progesterone prevented parturition occurring at about the expected time in about two-thirds of animals that were pregnant. The neonates of both groups were normal in size and weight, but about a third of treated pregnant animals retained their fetuses or aborted. The retained fetuses were retarded in development.

Therefore, progesterone treatment had no influence on the duration of gestation, or parturition, in the tammar wallaby, but high progesterone concentrations may interfere with the normal course of development and birth in a proportion of treated animals.