. The comparative anatomy of the domesticated animals. Veterinary anatomy. THE FCETUS. 1011 ifh-:' are seen in the middle layer, the vessels extending to the surface of the um- bilical vesicle, which gradually shows itself. While the contents of the vitel- line vesicle are undergoing absorption by the embryo, the heart is being completed, the vessels are developing, the allantois is formed, and the placental circulation— which continues until birth—is established. From this time the circulatory apparatus has acquired its definitive disposition. Fig. 575. 1. Appearance of the heart.—Circu- lati
. The comparative anatomy of the domesticated animals. Veterinary anatomy. THE FCETUS. 1011 ifh-:' are seen in the middle layer, the vessels extending to the surface of the um- bilical vesicle, which gradually shows itself. While the contents of the vitel- line vesicle are undergoing absorption by the embryo, the heart is being completed, the vessels are developing, the allantois is formed, and the placental circulation— which continues until birth—is established. From this time the circulatory apparatus has acquired its definitive disposition. Fig. 575. 1. Appearance of the heart.—Circu- lation in the umbilical vesicle.—The pleuro-peritoneal cavity of the embryo presents, anteriorly, a diverticulum— the cardiac cavity—in the interior of which the heart is developed. From what has been already said, it will be understood that the circu- latory apparatus is adapted to take the part, successively, of the umbilical vesicle and the placenta ; and from this two great systems arise—the umbilical or omphalo-mesenteric circulation, and the allantoid circulation. As Dareste has it, the heart is primarily double. Its two portions are formed on each side by a folding in- wards of the fibro-intestinal lamina and its endothelial lining. The two cardiac cavities open into each other by the absorption of the walls joined as a septum, and the simple heart assumes the form of a looped tube inclined to the right, as in Fig. 577. As soon as it shows itself, the heart contracts and dilates alternately, the movements being very slow, though they gradually become quicker. To- wards the twelfth day, the central organ of the circulation has the form of a contractile cylindrical tube. From its anterior part spring two branches—the aortic arches {arcus aortce)—which are directed towards the head of the embryo, and are afterwards inflected downwards and backwards. The aortic arches join together to constitute the single aorta, which, in its turn, divides into two trunks—th
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