. The comparative anatomy of the domesticated animals. Veterinary anatomy. J018 ~^ EMBRYOLOGY. to the envelopes sniToundiiig it, and which ramify in the placental tufts (Fig. 556). 1. The Chorion (Figs. 554, 555). The outermost envelope of the ovum, the chorion is a vast membranous and perfectly closed sac, the shape of which exactly corresponds with that of the uterus. It has a hodij and two cornua. The latter, when inflated, are plicated and sacculated like the caecum, and are always unequal in volume—that in which the foetus is developed having by far the largest dimensions.^ The external f
. The comparative anatomy of the domesticated animals. Veterinary anatomy. J018 ~^ EMBRYOLOGY. to the envelopes sniToundiiig it, and which ramify in the placental tufts (Fig. 556). 1. The Chorion (Figs. 554, 555). The outermost envelope of the ovum, the chorion is a vast membranous and perfectly closed sac, the shape of which exactly corresponds with that of the uterus. It has a hodij and two cornua. The latter, when inflated, are plicated and sacculated like the caecum, and are always unequal in volume—that in which the foetus is developed having by far the largest dimensions.^ The external face is studded with small red tubercles, formed by the placental tufts. It adheres to the internal surface of the uterus. Between the two membranes there is found a small quantity of sanguinolent fluid; but on one occasion, in the Ass, it had the appearance of milk. The internal face, lined by the external layer of the allantois, is united in the closest manner to that membrane, except at the umbilical cord, where there Fie. 555. EXTERIOR OF THE CHORIAL SAC (MARE). A, Body; B, C, cornua. exists a kind of conical infundibulum occupied by the umbilical vesicle. Ad- hesion is established by means of the vessels of the umbilical cord passing into the texture of the chorion, and by very delicate, laminal, interannexial connective tissue. Structure and Development.—The external covering of the ovum is at first formed by the vitelline tunic, itself constituted by the transparent membrane, covered externally by a layer of cells derived from the Fallopian tube or uterus. This primitive chorion is replaced by a portion of the extra-foetal somatopleure, which becomes the secondary or definitive chorion. It is destitute of vessels until the allantois is developed. When the chorion is complete, it is composed of three layers : 1. An external epithelial covering, formed of a single layer of uniform cylindrical cells, contain- ' When there is only one foetus, it is always more particularl
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