Memoirs: Notes on the Structure and the Development of the Elephant's Placenta

Abstract

1. The full-term after-birth of the elephant consists of a chorion from which spring many much-branched villi, which spread out in all directions into plate-like branches. These end in (a) proximal foliaceous terminations, in which the fœtal blood vessels ramify, which interlace with a complicated system of much larger blood channels filled with maternal blood, having well-defined but non-nucleated walls; (b) more distal lobate terminations, which are covered by a wellmarked columnar or cubical epithelium -- presumably the trophoblast -- which are partly embedded in a kind of coagulum or detritus, and partly appear to hang loosely in irregular blood spaces without walls ; (c) the stems of still more prolonged villi, which have been torn off and probably left embedded in the walls of the uterus; (d) a few torn ends of blood-vessels. 2. The main trunks of the villi and their foliaceous terminations are everywhere separated from the maternal bloodchannels by a syncytial layer, which is continuous with the epithelium covering the lobate terminations, and is presumably trophoblastic. 3. The half-term placenta originally examined by Owen in 1850 shows, in its more central region, characters which are essentially similar to those of the full-term specimen, and goes far to prove the existence of longer villi which penetrate deeply into the uterine mucosa. The lateral areas of the zonary belt exhibit many most interesting previous conditions. We are able to see in these the simple terminations of the foetal villi covered with a single layer of trophoblast separated from the uterine tissues by a layer of matei'ial partly maternal and partly of foetal origin. There is no process of growth round existing maternal capillaries to form an angio-plasmode, nor apparently any phagocytic action on the part of the trophoblast. The vascularisation of the after-birth is effected by the invasion of the trophoblast by extravasated maternal blood, which flows at first in intercellular and intervillous passages which form the larger channels of the after-birth maternal vascular system, and then makes its way along intra-cellular or intrasyncytial canals through a plasmodium produced by the breaking down of the trophoblast of two adjoining villi. We think the evidence is in favour of considering the corpuscles floating in this invading stream, which contains no red non-nucleated corpuscles in its more advanced portions, to be of maternal rather than trophoblastic origin. 4. The tissues of the full-term placenta contain pigment granules, which are deposited chiefly in the syncytial layer. This we regard as an excretory product; it is almost quite absent from the tissues of the half-term specimen. Leucocytes, either of maternal or foetal origin, seem to be concerned in the transference of this pigment into the maternal blood stream. 5. The subcircular bodies of Owen we find as described by him and Turner, though we note the presence of minute villi on their outer surface. 6. We confirm the opinion of previous writers that the zonary band in part is a "deciduous" form of placenta, although there is not much maternal tissue except the blood. It is not correct to speak of the after-birth being composed of a "much hypertrophied mncosa layer of the uterus." 7. The placenta of the elephant shows by its long villi, which tend to remain embedded in the uterus wall, a resemblance to the condition found in the Sirenia; by the villous patches at the poles and other villi which come out from the uterus, either with or without their trophoblastic covering, but with no maternal cells attached, a resemblance to the ungulata vera of the Perissodactyl type ; by the invasion of the trophoblast--if such it is--by the maternal blood stream, a resemblance to the Discoplacental type, although the actual manner by which this invasion occnrs would seem to be--so far as our very limited material affords us opportunity of observation--unlike anything hitherto described.1 8. The resemblance, at first sight obvious enough to the zonary placenta of the carnivora, is superficial. The elephant's placenta differs from that of the carnivora in (a) consisting of three areas of attachment instead of one, two of which, are wholly in the non-deciduous type, the other partly deciduous, partly non-deciduous. (b) There is nothing formed comparable to an angio-plasmode. (c) The maternal capillaries do not directly become the maternal vessels of the after-birth.