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Uterine specimens from fetal and postnatal heifers were examined by various types of microscopy to determine when the endometrium differentiated into caruncular and intercaruncular regions and changes in the cell types as the uterus developed. The luminal surface of the fetal uterus was formed into prominent pedunculated or mushroom-shaped nodules. Glandular development was first observed at about 250 days of gestation as short invaginations of the epithelium of the internodular surface adjacent to nodular stalks. The basal profile of each epithelial cell in the deeper portions of developing glands formed cytoplasmic processes which extended into the stroma. Between birth and 3 months of age there was a tremendous increase in the glandular epithelium. Concurrent thickening of the connective tissue component caused expansion of the nodular peduncle and a gradual flattening of the nodule until, by 1 year of age, only slight elevations remained, with a discrete caruncle visible on each. By following the development of the glandular component it was concluded that only the apical portion of each fetal nodule will develop into a caruncle and the sides of the nodule, together with the internodular portion, are destined to be the intercaruncular region.

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A study of the placenta of the spotted hyena (Crocuta croczrta Erxleben), based on five specimens from early in the second third of pregnancy to full term, is presented. Resembling the dog and cat in the retention to term of a large allantois and a medium-sized yolk sac, the hyena is unique among Carnivora in the possession of a hemochorial villous placenta, allied structurally more to that of New World monkeys than to the other carnivores. The intimacy of the fetal and maternal circulations is further increased by so-called intraepithelial capillaries in the syncytial trophoblast. The hyena, unlike most carnivores, possesses no central hematoma or marginal deposit of pigment, and lacks specialized structures in the chorionic membrane, The cytotrophoblast, however, does undergo modifications associated with phagocytosis of histotrophe in the paraplacental regions and in the junctional zone of the placenta proper.The principal mode of entry of maternal blood is directly into the subchorial sinus, into which uterine arteries, carried up in a crescent of gestational endometrial tissue at the margin of the placenta, empty. Smaller arteries reach the subchorial sinus in maternal septa that traverse the entire trophoblastic zone. Major arteries do not discharge blood at the base of the placenta, but basal drainage constitutes the sole means of return of blood to the uterine veins. The phylogenetic significance of the morphogenesis and vasculature of the placenta of Crocuta is discussed.Despite the common occurrence of the spotted hyena in the Old World tropics and the unrestricted hunting of the species permitted by the African authorities, a comprehensive account of the development and detailed structure of the placenta has hitherto been unavailable. The classical description by Matthews ('39) of the reproductive tract of Crocuta crocuta gives no information regarding placentation, and the more recent report by Morton ('57) is an account of the placenta and membranes based on a single, fullterm specimen. The remaining information on this subject is confined to a brief note by Matthews ('54), and to the observations of Amoroso ('55, '59a, '59b, '60, '61), which stimulated the present investigation.Interest in the placenta of the spotted hyena derives from two principal sources. Firstly, the histological differences between the placentation of this genus and that of the carnivores in general present an interesting basis for reconsidering the phylogenetic significance of the features of placental structure to which Mossman ('37) drew attention. Secondly, the intraplacental vascular relationships in Crocuta represent a striking example of perfection in countercurrent flow, the significance of which was first pointed out by Mossman ('26). In the present investigation, a morphogenetic approach is presented, with particular emphasis placed on the maternal circulation and its relationship to that in man and the other Primates. MATERIALS AND METHODSThe present study is based on the placentas and accessory organs of fi...

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FOURTEEN FIGURESThe placenta of the pig is ordinarily described as a simple apposition of chorionic and endometrial epithelia. Erosion of the endometrium and invasion by the chorion do not occur, although these processes are common in the placentas of many other mammals. We have been interested for many years in the comparative anatomy, histology and cytology of placental structures. Recently, with the availability of methods permitting application of the high resolution of the electron microscope to tissue sections, we have renewed our studies of placentas from several species, hoping that the newer details obtainable by electron microscopy would throw light on many problems still obscure. This paper represents the first of several dealing with the submicroscopic structure of placentas. We have chosen the pig as the first species to bc explored because the topography of its membranes is relatively simple, thus permitting ready orientation and correlation of the ultramicroscopic #details described here with our previous accounts of the histological and cytological organization of its placenta.This investigation was aided, in part, by grants S o . C 9 1 7 1 :and Yo. 0 2298,

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