956 SPECIAL PHYSIOLOGY. 



lime, and lastly, sebaceous matter, epidermoid scales, and minute hairs 

 thrown off from the embryo. 



The amnion, considered as an appendage of the embryo, is chiefly 

 protective, but may be respiratory or emunctory. It is present not 

 only around the embryo of the Bird, but also around that of Mamma- 

 lia and Reptiles. The embryos of the Amphibia and the Fish, how- 

 ever, which are developed in and surrounded by water, are destitute of 

 an amnion, no such covering being formed, and no folding over of the 

 external blastodermic layer taking place, as in the ovum of the Rep- 

 tile, Bird, and Mammal. 



The umbilical vesicle or yolk-sac already mentioned, may also be 

 regarded as another appendage of the embryo. It is formed, as just 

 described, by the lower part of the yolk-sac, surrounded by an exten- 

 sion of the innermost germinal layer, and connected with the intes- 

 tinal canal of the embryo by the constricted passage named the vitel- 

 line duct (Fig. 121). It is not only external to the body of the embryo, 

 but also outside the cavity of the amnion. Very early, certain blood- 

 vessels are developed in the middle germinal layer of the embryo, and, 

 spreading into a network of vessels, they extend upon the surface of 

 the yolk, and form the so-called vascular area. Two special branches, 

 named the omphalo-mesenteric arteries, convey blood from the embryo 

 through this area, and a vascular membrane is thus formed, which 

 gradually covers the umbilical vesicle or yolk-sac, and also extends 

 itself, at least in .the Birds' and Reptiles' eggs, by numerous projecting 

 folds, into the substance of the yolk. From the substance of the yolk, 

 these bloodvessels absorb dissolved nutrient material ; and, for a time, 

 the blood contained in them, is aerated by interchanges of carbonic 

 acid and oxygen, between it and the other fluids of the egg or ovum. 

 In the Bird, the yolk-sac is very large ; it is gradually drawn into the 

 abdomen of the chick, and may be found in that cavity after the chick 

 is hatched. In time, its contents are gradually absorbed, and its 

 remains are frequently traceable as a short blind sac or stem, the vitel- 

 line ccecum, connected with some part of the small intestine. Similar 

 changes are observed in Reptiles. In the Plagiostomatous Fishes, as 

 in the sharks, the yolk-sac remains, for a long time, suspended to the 

 abdomen of the young fish. In the Osseous Fishes, the yolk being 

 smaller, soon disappears. In the Cyclostomatous Fishes, and in the 

 Amphibia which have holoblastic ova, the yolk-sac is still smaller and 

 more transitory. Lastly, in the Mammalia, it is very minute, but 

 undergoes slight growth, especially in the Carnivora and Rodentia. It 

 is found, after a time, as a circular, pale-yellow disc, attached to the 

 amnion, and having no further function ; sometimes, as in Ruminantia, 

 it is completely absorbed. 



Besides the amnion and the yolk-sac, another most important ap- 

 pendage of the embryo in Birds, Reptiles, and Mammalia, remains to 

 be described. This is the so-called allantois. In the Bird's egg it 

 appears as a small eminence, at first consisting of nucleated cells, but 

 soon becoming vascular, situated on the under side of the embryo 

 close to its caudal end. It is derived from portions of the internal and 

 middle germinal layers, which, in this situation, form the intestinal 



