. Animal physiology. Physiology, Comparative; Physiology, Comparative. Fig. 286.—Formation of the digestive FORMATION OF DIGESTIVE CAVITY OF EMBRYO. 563 bation, its layers begin to exhibit various folds, which afterwards serve for the formation of the various cavities of the body. The parts of it which lie beyond the extremities, and which spread out from the sides of the embryo, are doubled in, so as to make a depression upon the yolk; and their folded edges gradually approach one another under the abdomen, which lies next the interior of the egg. In this e manner is formed the permanent dige
. Animal physiology. Physiology, Comparative; Physiology, Comparative. Fig. 286.—Formation of the digestive FORMATION OF DIGESTIVE CAVITY OF EMBRYO. 563 bation, its layers begin to exhibit various folds, which afterwards serve for the formation of the various cavities of the body. The parts of it which lie beyond the extremities, and which spread out from the sides of the embryo, are doubled in, so as to make a depression upon the yolk; and their folded edges gradually approach one another under the abdomen, which lies next the interior of the egg. In this e manner is formed the permanent digestive cavity; which is at first a simple pouch communi- cating with the yolk-bag, by a wide OJ)ening, as Seen at S, Fio*. cavity, e, embryo; /, g, layers of germinal no/3 -Li i • i t 11 membrane; h, heart ; s, stomach. zoo; but which is gradually separated from it by the narrowing of this orifice (as seen at s, Fig. 287), and becomes much more complex in its form. Thus we may say that the digestive cavity in Yertebrata is formed by the pinching-off (as it were) of a small portion of the general sac of the yolk. In the Mammalia, the remainder of the yolk-bag is completely separated from this, by the closure of its narrow orifice; and it is afterwards thrown off; so that only a very small portion of the germinal membrane is received into the per- manent structure. But in Birds and other oviparous animals, the whole of the yolk-bag is ultimately drawn into the abdomen of the embryo ; the former gradually shrinking, as its contents are exhausted ; and the latter enlarging, so as to receive it as a little pouch or appendage. In Fishes, the hatching of the egg very commonly takes place before this process has been completed ; so that the little Fish swims about with the yolk-bag hanging from its body. 749. The embryo, like the adult, has need of Respiration ; partly that its own heat may be kept up; .and partly that the carbonic acid liberated in the various processes of nutritio
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