[87] Numerous contributions to our knowledge of the various types of placenta have been made during the last few years, amongst which those of Turner and Ercolani may be singled out, both from the variety of forms with which they deal, and the important light they have thrown on the structure of the placenta. Amongst the existing Mammals provided with a true placenta, the most primitive type is probably retained by those forms in which the placental part of the chorion is confined to a comparatively restricted area on the dorsal side of the embryo; while the false chorion is formed by the vascular yolk-sack fusing with the remainder of the subzonal membrane. In all the existing forms with this arrangement of foetal membranes, the placenta is deciduate. This, however, was probably not the case in more primitive forms from which these are descended[88]. The placenta would appear from Ercolani's description to be simpler in the mole (Talpa) than in other species. The Insectivora, Cheiroptera, and Rodentia are the groups with this type of placenta; and since the rabbit, amongst the latter, has been more fully worked out than other species, we may take it first. [88] _Vide_ Ercolani, No. 197, and Harting, No. 201, and also Von Baer, _Entwicklungsgeschichte_ table on p. 225, part I., where the importance of the limited area of attachment of the allantois as compared with the yolk-sack is distinctly recognised. The Rabbit. In the pregnant female Rabbit several ova are generally found in each horn of the uterus. The general condition of the egg-membranes at the time of their full development is shewn in fig. 148. [FIG. 148. DIAGRAMMATIC LONGITUDINAL SECTION OF A RABBIT'S OVUM AT AN ADVANCED STAGE OF PREGNANCY. (From Kölliker after Bischoff.) _e._ embryo; _a._ amnion; _a._ urachus; _al._ allantois with blood-vessels; _sh._ subzonal membrane; _pl._ placental villi; _fd._ vascular layer of yolk-sack; _ed._ hypoblastic layer of yolk-sack; _ed´._ inner portion of hypoblast, and _ed´´._ outer portion of hypoblast lining the compressed cavity of the yolk-sack; _ds._ cavity of yolk-sack; _st._ sinus terminalis; _r._ space filled with fluid between the amnion, the allantois and the yolk-sack.]