Outlines of comparative physiology touching the structure and development of the races of animals, living and extinct : for the use of schools and colleges . rst rudiments of the eye appear, in the formof a fold in the external membrane of the germ, in which thecrystalline lens (fig. 307, x) is afterwards formed. At the sametime we see at the posterior part of the head an elliptical vesicle, DEVELOPMENT OF THE YOUNG WITHIN THE EGG. LSf) which is the rudiment of the ear. At this period, the dis-tinction between the upper and the lower layer of the germ isbest traced ; all the changes mentioned
Outlines of comparative physiology touching the structure and development of the races of animals, living and extinct : for the use of schools and colleges . rst rudiments of the eye appear, in the formof a fold in the external membrane of the germ, in which thecrystalline lens (fig. 307, x) is afterwards formed. At the sametime we see at the posterior part of the head an elliptical vesicle, DEVELOPMENT OF THE YOUNG WITHIN THE EGG. LSf) which is the rudiment of the ear. At this period, the dis-tinction between the upper and the lower layer of the germ isbest traced ; all the changes mentioned above appertaining tothe upper layer. § 467. After the seventeenth day, the lower or mucous layerdivides into two sheets, the inferior of which becomes the in-testine ; the heart shows itself about the same time, under theform of a simple cavity (fig. 307, /?), in the midst of a mass ofcells belonging to the middle or vascular layer. As soon as thecavity of the heart is closed in, regular motions of contractionand expansion are observed, and the globules of blood areseen to rise and fall in conformity with these motions. Fig. 306. Fig 307. Ficr. § 468. There is as yet, however, no circulation. It is notuntil the thirtieth day that its first traces are manifest in theexistence of two currents, one running towards the head, theother towards the trunk (fig. 308), with similar returning cur-rents. At this time the liver begins to form. Meanwhile theembryo gradually disengages itself at both extremities from itsadherence to the yolk ; the tail becomes free, and the younganimal moves it in violent jerks. § 469. The embryo, although still inclosed in the egg, nowunites all the essential conditions for the exercise of the func-tions of animal life. It has a brain, an intestine, a pulsatingheart and circulating blood, and it moves its tail spontaneously ;but the forms of the organs are not yet complete, nor havetliev acquired the precise shape characterizing the class, thefamil
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectzoology, bookyear1870
