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THE EVOLUTION OF MAN. 



only in the three higher classes of Vertebrates. It grows 

 from the hind end of the intestinal canal, from the pelvic 

 intestinal cavity (Figs. 133, 1, 135, r, u, 136, p, 139, al). Its 



r cf 



t 7i 7i vo 7 J 3 



Fig. 135. — Longitudinal section through the embryo of a Chick (in the 

 fifth day of incubation). The embryo with curved dorsal surface (black) : 

 d, intestine ; o, mouth ; a, anus ; I, lungs ; 7;, liver ; g, mesentery ; v, aui'icle ; 

 h, ventricle ; h, arterial arches ; t, aorta ; c, yelk-sac ; m, yelk-duct ; iv, 

 allantois ; r stalk of allantois ; n, amnion ; ic, amnion-cavity ; s, serous 

 membrane. (After Baer.) 



first rudiment appears as a small vesicle on the edge of the 

 pelvic intestinal cavity, representing an extension of the 

 intestine, and therefore (like the yelk-sac) has a two-layered 

 wall. The cavity of the vesicle is coated by the intestinal- 

 glandular layer, and the outer lamella of the wall is formed 

 by the thickened intestinal-fibrous layer. The small vesicle 

 grows larger and larger, and forms a sac of considerable size, 

 filled with liquid, and in the wall of which large blood- 

 vessels form. It soon reaches the inner wall of the egg- 



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