The ultrastructure of four roe-deer blastocysts at different stages of embryonic development were studied. During delayed implantation, the outer surface of the trophoblast possessed numerous microvilli and periodic invaginations or caveolae. There was a marked lack of organelles such as mitochondria or endoplasmic reticulum in the cytoplasm of the trophoblast cells, though many lipid droplets, granular inclusions and a lamina of fine fibrillae were present. Elongation of the blastocyst was associated with a decrease in the size and number of the microvilli, the disappearance of lipid droplets and granular inclusions, a reduction in the amount of fibrillar material and a dramatic increase in the development of mitochondria, granular endoplasmic reticulum, ribosomes and Golgi apparatus.The histology of the ovaries and uterus was studied in thirty-one roe deer. No prominent changes occurred in the ovaries at any stage of development; all ovaries possessed active CL and showed signs of follicular growth and atresia. Changes in the degree of mitotic activity, epithelial cell height, endometrial vascularity and stromal oedema were observed in the uterus throughout the period of delayed implantation and during the phase of rapid embryonic growth. Elongation of the embryo was associated with a marked decline in the height of the glandular epithelium and an increase in endometrial vascularity.The most important ultrastructural changes in the uteri of six roe deer were observed in the endometrial glands. Delayed implantation was associated with the accumulation of numerous supranuclear vesicles derived from the Golgi apparatus, while the resumption of embryonic growth was correlated with their sudden disappearance. When elongation had

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In early pregnancy the equine placenta consists of a simple apposition of fetal and maternal epithelia, but it becomes more complex with the formation of microcotyledons between 75 and 100 days of gestation. Although the placental barrier maintains an epitheliochorial arrangement throughout the course of pregnancy, a thinning of the maternal epithelium and a progressive indentation of the chorionic epithelium by fetal capillaries shortens the length of the diffusion pathway and reduces the amount of placental tissue between fetal and maternal bloodstreams. These structural modifications may reflect the changing requirements of the fetus for O2 and other metabolites as gestation proceeds. During the first 200 days of pregnancy there is evidence of intense pinocytotic activity by the cells of the trophoblast. From the 100th day of pregnancy there is a pronounced development of smooth endoplasmic reticulum, while rough endoplasmic reticulum and irregular, dense, membrane-bound bodies are a prominent feature of the paranuclear cytoplasm from Day 200. These changes suggest that the cells of the trophoblast become more highly involved in synthetic processes with increasing gestational age.

Placental separation in the ewe appears to involve an explosive degeneration of the chorionic epithelium which occurs in the short space of time between parturition and the delivery of the fetal membranes. In the goat there is a similar mechanism of separation. By contrast separation of the placenta in the cow appears to depend upon a detachment at the junctional region between fetal and maternal epithelia. There is no obvious explanation for these findings. The results are discussed in the light of the placental hormonal environment in the period which immediately precedes parturition.The placenta of the ewe, like that of other ruminants, is divided into cotyledonary and intercotyledonary areas. The cotyledons consist of large numbers of highly branched villi, which interdigitate with corresponding crypts in the uterine caruncles to form an epitheliochorial placenta [Lawn, Chiquoine and Amoroso, 1969). In the ewe the cotyledons are concave and sessile: the fetal vessels enter the substance of the cotyledon from the concavity of the central depression [Steven, 1968]. From the 8th week of gestation this central region of the cotyledon is characterized by an extravasation of maternal blood, which separates the tips of the maternal septa from the arcades of chorionic epithelium at the bases of the chorionic villi [Wimsatt, 1950]. In the zona intima of the deeper regions of the cotyledon, fetal and maternal epithelia are held together by an interdigitating microvillous junction. There are no marked differences in placental morphology between the ewe and the goat.The general arrangements of the bovine placenta are similar to those of the other two species. In the cow, however, the cotyledons are convex and pedunculated, the fetal vessels are distributed over the convex surface of the cotyledons and there is no obvious extravasation of maternal blood at the tips of the maternal septa. In cattle retention of the placenta following parturition is a common clinical condition [Bj6rkman and Sollen, 1961] which disrupts breeding programmes and leads to considerable economic loss. In sheep and goats retention of the placenta is very rare indeed.It has been shown by Bj6rkman and Soll6n [1960] that under normal circumstances separation of the bovine placenta occurs within the cotyledons at the line of microvillous attachment between fetal and maternal epithelia, and that the process of separation involves a loosening of the attachment between fetal and maternal microvilli with little damage either to fetal or maternal tissues. In this paper evidence is presented to show that placental separation in the sheep and goat is achieved by a very different mechanism. A preliminary report has already been published [Steven, 1973].

Extravasated maternal blood, which escapes from capillaries and larger blood vessels within the tips of the maternal septa, is responsible for the characteristic pigmentation of the central depression of the ovine cotyledon in the last third of pregnancy. The chorionic epithelium of this region is actively engaged in the uptake and subsequent breakdown of maternal erythrocytes, which may represent an important source of iron for the foetus during the period of maximum intra-uterine growth.Haemophagous areas of the paraplacental chorion are a characteristic feature of the placenta of most carnivorous mammals [Amoroso, 1952]: in such areas the cells of the chorion appear to be actively engaged in the uptake and subsequent breakdown of extravasated maternal blood [Amoroso, 1955]. Such haemophagous areas have been studied in the cat and dog