. Principles of zoölogy : touching the structure, development, distribution, and natural arrangement of the races of animals, living and extinct with numerous illustrations : Part 1, Comparative physiology : for the use of schools and colleges. Fig. 126. amnios, in another large sac. The tubular part of this sac, which isnearest the embryo, is at last transformed into the urinary heart {h) is akeady very large, with mniute arterial threads DEVELOPMENT OF THE YOUNG WITHIN THE EGG. 151 the water, with the well-protected eggs of birds, andstill more with the growth of young mamm
. Principles of zoölogy : touching the structure, development, distribution, and natural arrangement of the races of animals, living and extinct with numerous illustrations : Part 1, Comparative physiology : for the use of schools and colleges. Fig. 126. amnios, in another large sac. The tubular part of this sac, which isnearest the embryo, is at last transformed into the urinary heart {h) is akeady very large, with mniute arterial threads DEVELOPMENT OF THE YOUNG WITHIN THE EGG. 151 the water, with the well-protected eggs of birds, andstill more with the growth of young mammals within thebody of the mother. 317. But neither in fishes, nor in reptiles, nor in birds,does the vitelline membrane, or any other envelope of the egg,take any part in the growth of the embryo; while on the. Fig. 127. Fig. 128. passing off from it. At this period there exist true gills upon thesides of the neck, and a branchial respiration goes on. 315 d. The development of ^ mammals exhibits the followingpeculiarities. The egg is ex-ceedingly minute, almost micro-scopic, although composed of thesame essential elements as thoseof the lower animals. The vitel-line membrane, called chorion, in this class of animals, is comparatively thicker, (Fig. 127, v,) alwayssoft, surrounded by peculiar cells, being a kind of albumen. Thechorion soon grows proportionally larger than the vitelline sphereitself, (Fig. 128, y,) so as no longer to invest it directly, being sepa-rated from it by an empty space, (k.) The germ is formed in thesame position as in the other classes of Vertebrates, namely, at the topof the vitellus, ( ;) and here alsotwo layers may bedistinguished, the up-per or serous layer, (*,)and the low^er or mxi-cous layer, {in.) Asit gradually enlarges,the surface of thechorion beco
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