. Comparative zoology, structural and systematic. For use in schools and colleges. Zoology. 190 COMPARATIVE ZOOLOGY. iucli in diameter), inclosed in a sac, called the vitelline membrane, and disposed in concentric layers, like a set of vases placed one within the other. That part of the yolk which extends from the centre to a white spot {cicatricu- la) on the outside can not be hardened, even with the most prolonged boiling. The cicatricula, or embryo-spot —the part for which all the rest was made—i^ a thin disk of cellular structure, in which the new life first appears. It is always on that s
. Comparative zoology, structural and systematic. For use in schools and colleges. Zoology. 190 COMPARATIVE ZOOLOGY. iucli in diameter), inclosed in a sac, called the vitelline membrane, and disposed in concentric layers, like a set of vases placed one within the other. That part of the yolk which extends from the centre to a white spot {cicatricu- la) on the outside can not be hardened, even with the most prolonged boiling. The cicatricula, or embryo-spot —the part for which all the rest was made—i^ a thin disk of cellular structure, in which the new life first appears. It is always on that side which naturally turns uppermost,. Fio. 15S.—Longitudinal section of Hen's Egg before incubation: a, yolk, showing concentric layers; a', its semi-fluid centre, consistiug of a white granular sub- stance—the whole yolk is inclosed iu the vitelline membrane; h, inner dense part of the albumen ; 6', outer, thinner part; c, the chalazs, or albumen, twisted by the revolutions of the yolk; d, double shell-membrane, split at the large end to form the chamber/,' e, the shell; h, the white spot, or cicatricula, and under it the germinal vesicle of Purkinje, or nucleus, which is afterward ruptured, and becomes invisible. for the yolk can turn upon its axis; it is, therefore, al- ways nearest to the external air and to the Hen's bod}'— two necessary conditions for its development. There is another reason for this polarity of the egg: the ligliter and most delicate part of the yolk, the cicatricula, is col- lected where the upper cavity of the animal, inclosing the nervous system, is to be; while the heavy oily portion re- mains beneath, where the lower cavity, inclosing the or- gans of nutrition, is afterward developed. The essential parts of any egg are the germ-cell, or cic-. 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
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Keywords: ., bookcentury1800, bookdecade1880, bookpublishe, booksubjectzoology
