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M. L. Norris

Summary. Within the same female, concurrent lactation had no apparent effect on the development and hatching of embryos artificially retained in the oviduct while effectively inhibiting zona shedding and implantation in the contralateral uterine horn. This result is believed due to a specific inhibitor(s) present in the uterine fluid or to physical constriction by the uterus. These extrinsic, maternal factors act either singly or in combination to influence the mature blastocyst.

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M. L. Norris

Summary. The efficacy of several exteroceptive factors including exposure to a 'strange' male, were tested to induce pregnancy failure in newly mated Mongolian gerbils. Only physical separation of monogamous pairs consistently induced pregnancy failure. This phenomenon was restricted to the immediate period after mating and is believed to be due to disruption of pair bonding, with associated luteal and implantation failure. Concurrent lactation annulled this effect.

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M. L. Norris and W. F. Rall

Summary. Lactating pregnant gerbils in which one oviduct was ligated during early pregnancy were used on Day 6–14 p.c. as recipients for eggs (morulae and blastocysts) recovered on Day 6–12p.c. from lactating donors. Eggs were transferred to the ligated (experimental) horn and the litter was removed. About half of the recipients had implantations in the non-ligated (control) horn 15 days after litter removal. Of these 40 females, 37 had implantations in the experimental horn and 32 had some normal fetuses present. Of 179 eggs transferred into the ligated uteri, 61% implanted and 41% developed into normal fetuses. The time after copulation of recipient and donor did not appear to influence the number of implantations or post-implantation development of the transferred eggs.

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C. E. ADAMS and M. L. NORRIS

Since introducing the Mongolian gerbil to our laboratory in 1965, we have accumulated a variety of data concerning the reproduction of this species. The present report is based on the 4-year period, 1967-70.

Details of the origins and management of our colony have been given elsewhere (Norris & Adams, 1972a). Breeding stock were kept as monogamous pairs. If no litter resulted within 3 months of pairing, a different male was introduced and, whenever necessary, this procedure was repeated after a further 3 months. If the female still proved barren, autopsy was performed. Any males or females which showed a strong tendency to behave aggressively were culled. Routine procedures, such as inspection of cages for litters and taking vaginal smears, were carried out twice daily at 09.00 to 09.30 hours and 17.00 to 17.30 hours.

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M. L. Norris and C. E. Adams

Summary. A series of experiments based on 150 newly mated, uni- and multi-parous Mongolian gerbils provided no evidence for the existence of pregnancy blocking due to a strange male effect. The reduced level of fertility observed was attributed to stressful factors at critical times in the pregnancy. Treatment with 10 i.u. PMSG on the day following mating interrupted pregnancy and the majority of females mated again within 4 days.

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M. L. Norris and C. E. Adams

Summary. Observations were made on vaginal smear pattern, mating, rate of egg transport and preimplantation development in cuis. In 39 of 45 females (87%) vaginal opening occurred within 2 days of pairing, and in 44 (98%) within 5 days; 36 of the 44 females (82%) mated. By the morning of Day 2, eggs were no longer recovered from the ampulla, having all passed into the isthmus, where they remained till Day 4, p.m. Uterine entry was complete by Day 5, a.m. Uterine blastocysts were no longer recoverable on the morning of Day 6. There was no evidence of blastocyst expansion or zona shedding before implantation.

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M. L. NORRIS and C. E. ADAMS

In the Mongolian gerbil, vaginal opening occurs at 40 to 60 days of age (Nakai, Nimura, Tamura, Shimizu & Nishimura, 1960; Schwentker, 1963; Marston & Chang, 1965), whilst the age of sexual maturity has been given as 9 to 12 weeks (Schwentker, 1963; Marston & Chang, 1965). Recently, Norris & Adams (1972a) estimated that the mean age at first conception was 75 days under the most favourable conditions, namely 60-day-old females paired with sexually experienced males. Normal growth data from birth to 150 days of age have been presented by Norris & Adams (1972b). The present study was undertaken in order to obtain information on ovarian development in relation to puberty and sexual maturity in this species. Recently, Stark (1973) has reported on ovarian development in gerbils aged 0 to 21 days.

Observations were made on

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M. L. Norris and C. E. Adams

Summary. Ovarian follicles were counted and classified in control and unilaterally ovariectomized Mongolian gerbils, which had ceased to breed, at 306–1020 days and 340–902 days of age respectively. With increasing age there was a steady reduction in the non-antral follicle population, but a reduction in the number of antral follicles only occurred in extreme old age. Approximately twice as many non-antral follicles were present in control as in unilaterally ovariectomized females of similar ages, whereas the number of antral follicles was similar in both groups. There were no instances of total depletion of the follicle stock, the lowest numbers recorded being 288 and 74 in control and unilaterally ovariectomized females respectively.

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M. L. NORRIS and C. E. ADAMS

The Mongolian gerbil is normally a very tame and tractable animal. In certain situations, however, it may behave aggressively as, for example, when sexually mature animals are paired. Schwentker (1963) regarded the Mongolian gerbil as monogamous, observing that when old females lost their mate they did not usually accept another. Marston & Chang (1965) reported that polygamous mating groups were less successful than monogamous pairs and that careful supervision of mating groups was essential. They also noted that it was not possible to carry out test matings owing to the female's aggressiveness towards a strange male. In accord with these observations, Barfield & Beeman (1968), who made a study of the oestrous cycle, found it necessary to introduce each female to a specific male to avoid aggression during the non-receptive periods. The