File:Outlines of comparative physiology touching the structure and development of the races of animals, living and extinct - for the use of schools and colleges (1870) (14596177758).jpg

Identifier: outlinesofcompar00agas (find matches)
Title: Outlines of comparative physiology touching the structure and development of the races of animals, living and extinct : for the use of schools and colleges
Year: 1870 (1870s)
Authors: Agassiz, Louis, 1807-1873 Gould, Augustus A. (Augustus Addison), 1805-1866 Wright, Thomas, 1809-1884
Subjects: Physiology, Comparative Zoology
Publisher: London : Bell & Daldy
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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duringits growth. From one of these envelopes, the allantois (fig.311, a), is derived their common name of allantoidicm rer-tebrata, in opposition to the naked reptiles and fishes, whichare called anallantoidian. § 4/3. The allantoidian vertebrata differ from each otherin several essential peculiarities. Among birds, as well as inthe scaly reptiles, we find at a certain epoch, when the embryois already disengaging itself from the yolk, a fold rising aroundthe body from the upper layer of the germ, so as to present, ina longitudinal section, two prominent walls (fig. 310, x, x).These walls, converging from alJ sides upwards, rise graduallytill they unite above the middle of the back (fig. 311). Whenthe junction is effected, which in the hens egg takes place inthe course of the fourth day, a cavity is formed betwreen theback of the embryo (fig. 312, e) and the new membrane,whose walls are called the amnios. This cavity becomes filledwith a peculiar liquid, the amniotic water. Fi?. 312.
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§ 4/4. Soon after the embryo has been enclosed in theamnios, a shallow pouch forms from the mucous layer belowthe posterior extremity of the embryo, between the tail andthe vitelline mass. This pouch, at first a simple little sinus(fig. 311, «), grows larger and larger, till it forms an extensivesac, the allantois turning backwards and upwards, so as com-pletely to separate the two plates of the amnios (fig. 312, a),and finally enclosing the whole embryo, with its amnios, in 288 EMBRYOLOGY. another large sac. The tubular part of this sac, which isnearest the embryo, is at last transformed into the urinarybladder. The heart (k) is already very large, with minutearterial threads passing off from it. At this period thereexist true gills upon the sides of the neck, and a branchialrespiration goes on. $ 475. The development of mammals exhibits the followingpeculiarties : the egg is exceedingly minute, almost microsco-pic, although composed of the same essential elements asthose of the lo

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