The embryo is surrounded by the amnion, which is comparatively small. The yolk-sack (_ds_) is large and attached to the embryo by a long stalk. It has the form of a flattened sack closely applied to about two-thirds of the surface of the subzonal membrane. The outer wall of this sack, adjoining the subzonal membrane, is formed of hypoblast only; but the inner wall is covered by the mesoblast of the area vasculosa, as indicated by the thick black line (_fd_). The vascular area is bordered by the sinus terminalis (_st_). In an earlier stage of development the yolk-sack had not the compressed form represented in the figure. It is, however, remarkable that the vascular area never extends over the whole yolk-sack; but the inner vascular wall of the yolk-sack fuses with the outer, and with the subzonal membrane, and so forms a false chorion, which receives its blood supply from the yolk-sack. This part of the chorion does not develop vascular villi. The allantois (_al_) is a simple vascular sack with a large cavity. Part of its wall is applied to the subzonal membrane, and gives rise to the true chorion, from which there project numerous vascular villi. These fit into corresponding uterine crypts. It seems probable, from Bischoff's and Kölliker's observations, that the subzonal membrane in the area of the placenta becomes attached to the uterine wall, by means of villi, even before its fusion with the allantois. In the later periods of gestation the intermingling of the maternal and foetal parts of the placenta becomes very close, and the placenta is truly deciduate. The cavity of the allantois persists till birth. Between the yolk-sack, the allantois, and the embryo, there is left a large cavity filled with an albuminous fluid. The Hare does not materially differ in the arrangement of its foetal membranes from the Rabbit. [FIG. 149. SECTION THROUGH THE PLACENTA AND ADJACENT PARTS OF A RAT ONE INCH AND A QUARTER LONG. (From Huxley.) _a._ uterine vein; _b_. uterine wall; _c._ cavernous portion of uterine wall; _d._ deciduous portion of uterus with cavernous structure; _i._ large vein passing to the foetal portion of the placenta; _f._ false chorion supplied by vitelline vessels; _k._ vitelline vessel; _l._ allantoic vessel; _g._ boundary of true placenta; _e_, _m_, _m_, _e_. line of junction of the deciduate and non-deciduate parts of the uterine wall.]