. Animal physiology. Physiology, Comparative; Physiology, Comparative. 562 NUTRITION OF THE incubation, as a simple dilatation of the trunk into which the blood vessels unite (Fig. 286, h). Its walls are at first formed by a layer of cells; and no muscular structure is seen in it, until after its regu- lar pulsations have com- menced. The mode in which the nourishment of the em- bryo is imbibed, through the vascular area, from the store laid up in the yolk, bears a striking analogy with that, in which the embryo of the Flowering Plant draws in the store, laid up in its cotyledons or i
. Animal physiology. Physiology, Comparative; Physiology, Comparative. 562 NUTRITION OF THE incubation, as a simple dilatation of the trunk into which the blood vessels unite (Fig. 286, h). Its walls are at first formed by a layer of cells; and no muscular structure is seen in it, until after its regu- lar pulsations have com- menced. The mode in which the nourishment of the em- bryo is imbibed, through the vascular area, from the store laid up in the yolk, bears a striking analogy with that, in which the embryo of the Flowering Plant draws in the store, laid up in its cotyledons or in its albumen. It will be recollected that, in many Dicotyledons, the cotyledons absorb the starchy and oily matter of the albumen into their own substance, before the ripening of the seed ; so that the seed contains only the embryo and the fleshy cotyledons, the latter of which gradually impart their contents to the former. But in a large number of Dicotyledons, and all Monocotyledons, the albumen remains as a separate store, even when the seed is ripe ; and is afterwards absorbed by the double or single cotyledon ; and imparted to the embryo through its means, as in the Bird's ^gg- (Veget. Piiys. §. 440-442). The cells of the germi- nal membrane in the Animal, and those of the cotyledons in the Plant, probably serve to elaborate or prepare the nou- rishment, for the use of the germ. It is in the former, that the matter of the yolk is first converted into blood. 748. The germinal membrane surrounding the yolk, may still be regarded as the temporary stomach of the embryo; but instead of the whole . of it becoming the permanent digestive cavity, as in the Radiated classes, only a small part of it is retained for this purpose. At about the 25th hour of incu- Fig. —Embrvo of Bird, with the ves- sels (/) of the Vascular Area, aftkr four days' Please note that these images are extracted from scanned page images that may have been digitally enhanced for readabilit
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