Plasma progesterone was measured by radioimmunoassay in individual female Myotis lucifugus throughout pregnancy and lactation. Progesterone levels, which averaged 6.7 +/- 0.7 ng/ml in late hibernation, rose to a mean of 18.9 +/- 6.7 ng/ml in unimplanted bats collected in the first two weeks after arrival at a maternity roost. Analysis of progesterone levels in bats in which the developmental stage of the embryo was known revealed two sharp, transient increases in plasma progesterone during the preimplantation period. The first, with values of 30-45 ng/ml, occurred at ovulation. The second, with values of 20-30 ng/ml, coincided with blastocyst formation. Progesterone levels increased exponentially from a mean of 7.4 +/- 1.0 ng/ml during early implantation to peak values of 100-200 ng/ml (means = 136.2 +/- 15.6) in the last two weeks of pregnancy, and showed no evidence of either a midpregnancy or prepartum decline. Despite involution of the corpus luteum at the end of pregnancy, progesterone levels averaged 9.0 +/- 1.0 ng/ml during lactation and did not decline until the end of lactation. In bats undergoing abortion, mean levels of plasma progesterone were already less than 6 ng/ml, equivalent to levels in nonbreeding females. The results indicate that the progesterone profile of pregnant M. lucifugus, though generally resembling those of other bats, exhibits several distinctive features. The sharp rise in plasma progesterone coinciding with blastocyst formation has not been reported in other mammals and suggests a possible role of progesterone in the cavitation process. In addition, peak values of plasma progesterone in late pregnancy were conspicuously higher than levels reported in other verpertilionid bats. The levels did not appear to fall before parturition, although such falls have been reported in other bats.

The effect of ovariectomy during delayed implantation on the fate of the free blastocyst has been studied in the armadillo. If bilateral ovariectomy is performed about the middle of the 4-month delay period, implantation occurs 30-34 days later and is indistinguishable from normal implantation. In animals ovariectomized toward the end of the delay period, implantation occurs at the time when normal implantation would have been expected, which is less than 30 days postoperatively. However, resorption or abortion follows shortly thereafter. Removal of the corpus luteum towards the end of the delay period for the most part results in loss of the blastocyst and non-implantation. Removing the corpus luteum in early gestation causes loss of the embryos, but no effect was observed following loss of the corpus luteum in animals past approximately the first third of gestation. A possible explanation of the findings is discussed.The effect of loss of the luteal tissue, or loss of all the ovarian tissue, on the course of pregnancy has been studied in a number of species. The invariable result of such operations in early pregnancy is abortion or résorption of the embryos. In many species (e.g. man, monkey, cat, guinea-pig) ablation of the ovaries in the latter portion of pregnancy does not lead to foetal loss, while in others (e.g. rat, rabbit) the ovaries must be present until parturition, but this is generahy conceded to be due to the placental production of steroid hormones in the former species, which renders the ovaries expendable as a source of these secretions.The effect of ovariectomy prior to implantation (but necessarily after ovulation) has scarcely been investigated. The time interval in most species is short, usually 7-10 days, and since the maintenance of early pregnancy is dependent on the presence of functional corpora lutea, it seems improbable that implantation could be initiated in the absence of the ovaries. Animals exhibiting delayed implantation offer an extended period in which such operations can be performed. Furthermore, possible interference with other events preceding implantation, such as tubai passage, can be avoided easily.A few workers have examined the results of ovariectomy in the interval between ovulation and implantation in laboratory rodents. Loeb [1923] reported that removal of the corpora lutea of the guinea-pig in the first 48 hr following ovulation caused a failure to implant, but that removal after this time did not. He examined his animals 2 weeks after the operation and found normal embryos in the latter cases ; apparently, however, he did not ahow any animals to go to term. Loeb felt that in the first 48 hr after ovulation, the corpora lutea were able to sensitize the uterus suffi¬ ciently to make it receptive to the ova.

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Similar histological and histochemical changes occur in the uteri of pregnant and pseudopregnant ferrets. The pre-implantation period is characterized by general growth of the tissue and moderate secretory activity. At the time of implantation, the luminal epithelium undergoes hypertrophy and hyperplasia and shows much secretory activity. This area of altered activity sweeps progressively toward the gland fundi and the epithelium behind it becomes greatly enlarged and forms symplasmic masses. Symplasma formation occurs in all regions of the uterus, in contrast to other carnivores where it occiirs only next to trophoblast and in response to uterine trauma.The principal uterine secretions are mucoproteins and glycoproteins. There is some evidence that pseudopregnant uteri, but not pregnant uteri, secrete acid mucopolysaccharides. Glycogen is present in moderate amounts at estrus, declines during the pre-implantation period, and is absent during gestation. A light to moderate diffuse sudanophilia is seen during pregnancy and pseudopregnancy, becoming more pronounced toward the end of the gestation period. Occasional cholesterol crystals are seen in macrophages in the stroma of pregnant, but not pseudopregnant, uteri. Macrophages containing hemosiderin were seen in all uteri examined, being most abundant in estrous and early pre-implantation uteri. Uterine mast cells were abundant at estrus, declined during the pre-implantation period and decreased markedly at the time of implantation. The results are compared with data from other carnivore species.Several features make the ferret well suited for studies in reproductive physiology. There is a well defined breeding season and estrus is easily determined by the degree of vulval enlargement. Since the animals are induced ovulators, the various events of pregnancy can be timed rather accurately. Pregnancy lasts 40-42 days and consists of a relatively long preimpIantation period of 10-12 days and an active gestation period of 30 days. A period of pseudopregnancy which is exactly the same length as pregnancy follows sterile mating. The influence of the amount of daylight on the reproductive cycle has been much exploited in prior studies of the relationship of environment to neural control of reproduction. The basic features of the ferret reproductive cycle were first described by F. H. A. Marshall ('04). Although a more detailed description of the cycle and the histological changes of the reproductive tract during estrus, pseudopregnancy and pregnancy was published by Hammond and AM. J. ANAT., 118: 195-216.

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