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.
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 fœtal 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 fœtal 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 fœtal 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.