602 AMNION AND ALLANTOIS. 



By a continuation of the above process, the allantois at last 

 grows to such an extent as to envelope completely the body of the 

 embryo, together with the amnion ; its two 

 Fig. 210. extremities coming in contact with each 



other and fusing together over the back of 

 the foetus, just as the amniotic folds had 

 previously done. (Fig. 210.) It lines, there- 

 fore, the whole internal surface of the in- 

 vesting membrane with a flattened, vascu- 

 lar sac, the vessels of which come from the 

 interior of the body of the foetus, and which 

 with still communicates with the cavity of the 



allantois fully formed. a. Urn- intestinal Canal. 



b'.lical vesicle, b. Aranion. c. , , 



Aiiantois. It is evident, from the above description, 



that there is a close connection between the 



formation of the amnion and that of the allantois. For it is only 

 in this manner that the allantois, which is an extension of the in- 

 ternal layer of the blastodermic membrane, can come to be situated 

 outside the foetus and the amnion, and be brought into relation 

 with external surrounding media. The two laminas of the amni- 

 otic folds, in fact, by separating from each other as above described, 

 open a passage for the allantois, and allow it to come in contact 

 with the external membrane of the egg. 



In order to explain more fully the physiological action of the 

 allantois, we shall now proceed to describe the process of develop- 

 ment, as it takes place in the egg of the fowl. 



In order that the embryo may be properly developed in any 

 case, it is essential that it be freely supplied with air, warmth, 

 moisture, and nourishment. The egg of the fowl contains already, 

 when discharged from the generative passages, a sufficient quantity 

 of moisture and albuminous material. The necessary warmth is 

 supplied by the body of the parent during incubation ; while the 

 atmospheric gases can pass and repass through the porous egg- 

 shell, and by endosmosis through the fibrous membranes which 

 line its cavity. 



When the egg is first laid, the vitellus, or yolk, is enveloped in 

 a thick layer of semi-solid albumen. On the commencement of 

 incubation, a liquefaction takes place in the albumen immediately 

 above that part of the vitellus which is occupied by the cicatri- 

 cula ; so that the vitellus rises or floats upward toward the surface, 

 by virtue of its specific gravity, and the cicatricula comes to be 



