. Animal physiology. Physiology, Comparative. 584 DEVELOPMENT OP VESSELS AND DIGESTIVE CAVITY. first formed by a lay^r of cells; and no muscular structure is. seen in it, until after its regular pulsations have commenced. It is in these vessels that the first blood is formed; and the same process appears to be continued through the whole period of incubation, the yolk being progressively converted into blood, and this blood being conveyed by the great trunks which collect it into the body of the embryo. Looking at the yolk-bag in the light of a temporary stomach, its A^essels may be likened to
. Animal physiology. Physiology, Comparative. 584 DEVELOPMENT OP VESSELS AND DIGESTIVE CAVITY. first formed by a lay^r of cells; and no muscular structure is. seen in it, until after its regular pulsations have commenced. It is in these vessels that the first blood is formed; and the same process appears to be continued through the whole period of incubation, the yolk being progressively converted into blood, and this blood being conveyed by the great trunks which collect it into the body of the embryo. Looking at the yolk-bag in the light of a temporary stomach, its A^essels may be likened to those which take so large a share in the act of absorption from the digestive cavitv of the adult (§ 218). 759. During the same early period of incubation, the layers of the germinal membrane begin to exhibit various folds, which after- wards serve for the forma- tion of the several cavities of the body. The points 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 an- f ' other imder the abdomen, which lies next the in- terior of the egg. In this manner is formed the per- manent digestive cavity ; which is first a sim])le pouch comnmnicating Avith tlie yolk-bag, by a wide opening, as seen at s, fig. 320 ; but which is gradually separated from it by the narrowing of this orifice (fig. 322), the connecting- portion being elongated into a duct (fig. 321, h). Thus we may I. Fig. 319.—Embryo of Bird, with the Vessels, /, of the vascular area, alter lour days' incubation. ^ Fig. -DlAGRAM OF THE DIGESTI^ THF, Formation E Cavity: e, embryo; /, .9, layers of germinal mem brane; /i, heart; s, Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Carpenter, Willi
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Keywords: ., bookauthorcarpente, bookcentury1800, bookdecade1850, bookyear1859
