The implantation of the blastocyst of
Loxodonta
, and the early development of the placenta, are described from material collected in Africa by Dr R. M. Laws. Implantation is central, the blastocyst settling in one of three or four deep longitudinal grooves in the uterine lumen. Its growth distends the uterine horn before it becomes attached to the uterine wall. As the bilaminar blastocyst continues to grow the trophoblast erodes the uterine epithelium over a zonary area and is there closely applied to the uterine stroma. It remains unattached over the embryonal and abembryonal poles, where the uterine epithelium is retained. As the yolk-sac approaches its maximum volume it is invested by mesoderm, forming an extensive trilaminar omphalopleur, the outer layer (trophoblast) of which immediately begins to invade the uterine stroma by peg-like proliferations that enter by the uterine glands. The latter undergo little change during these early stages and do not undergo extensive modification at any stage, but their basal portions become moderately distended after the gland openings are blocked by the trophoblast. As the trophoblast of the yolk-sac wall invades the stroma the allantois reaches the chorion and from this time the yolk-sac is rapidly reduced in volume. The allantois soon fills the exocoel and occupies the whole cavity of the conceptus surrounding the embryo and amnion. The allantochorionic placenta develops (over three discrete areas in the specimen described) by the growth of villi which are formed as the earliest trophoblast proliferations acquire a mesenchymal core and become vascularized. There remain areas where the trophoblast does not attach to the uterine wall and in these areas the uterine epithelium proliferates in a characteristic manner and appears to shed cellular material into the residual uterine lumen. As the allantochorionic villi develop, the underlying uterine stroma thickens, and large blood channels appear in it, lined by a shallow endothelium. These blood vessels, which have few branches, penetrate to the face of the placenta. Their investment by the advancing trophoblast leads to the ‘ vasochorial ’ condition described in an earlier account. The characteristic marginal haematomata of the elephant apparently form where an extending villous area meets an area of intact (although modified) uterine epithelium. This epithelium is undermined by lateral extension of the invading foetal tissue and some of the adventitious maternal blood vessels that reach the face of the placenta are disrupted and release blood into the uterine lumen where the stromal tissue is exposed between the advancing foetal villi and the surviving uterine epithelium. This blood is trapped in folds of the allantochorion, the trophoblast cells of which often contain maternal erythrocytes. These developmental characters are discussed with reference to their functional significance, and compared with the corresponding changes in the Carnivora, most of which are also characterized by an endotheliochorial placenta of zonary form with haematomata, marginal or otherwise. It is suggested that their occurrence is related to the intermediate position of the endotheliochorial placenta between the epitheliochorial type, in which the uterine glands contribute more importantly to embryonic nourishment, and the haemochorial type, in which transfer from the maternal circulation to the trophoblast is facilitated by the direct contact between them. The mode of implantation is shown to be very different from that in hyrax, which superficially resembles
Loxodonta
in the morphology of the foetal membranes. Comparison is also made with the aard-vaak and the manatee. The aard-vaak has a zonary endothelial placenta, marginal haematomata and a quadrilocular allantois, but does not resemble
Loxodonta
closely in detailed placental structure. The mid-term placenta of the manatee, on the other hand, bears a very striking resemblance to that of the elephant in many respects, especially in the manner in which the trophoblast is modified where it invests large maternal blood vessels. The phylogenetic significance of these similarities and differences is briefly discussed.
Uterine Glands Provide Histiotrophic Nutrition for the Human Fetus during the First Trimester of Pregnancy