742 ACCESSORY ORGANS IN IMPREGNATED EGG. 



vessels of which come from the interior of the body of the embryo, and 

 which still communicates with the cavity of the intestinal canal. 



It is evident, accordingly, that there is a close connection between 

 the formation of the amnion and that of the 

 Fig. 258. allantois. For it is only by this means that 



the allantois, which is an extension of the in- 

 ternal blastodermic layer, can come to be 

 situated outside the embryo and the amnion, 

 and thus brought into relation with surround- 

 in or media. The two laminae of the amniotic 



O 



folds, by separating from each other as above 

 described, open a passage for the allantois, and 

 allow it to come in contact with the external 

 Diagram of the FECUNDA- membranous investment of the egg. 



TED EGG, with the allantois _. . , . _ . - , 7 . . mu 



fully formed. a. Umbilical Pnysiological Action of the Allantois. Ihe 



vesicle, b. Amnion. c. Allan- physiological action of the allantois, in its 



simplest form, may be studied with advantage 



in the fowl's egg, where it forms an extensive and highly vascular 

 organ, but does not present any important modifications of its original 

 structure. 



The egg of the fowl contains, when first laid, an abundant deposit of 

 semi-solid albuminous matter in which the yolk is enveloped. This 

 affords, in connection with the yolk, a sufficient quantity of moisture 

 and organic nutriment for the growth of the embryo. The necessaiy 

 warmth is supplied by the body of the fowl in incubation ; and the 

 atmospheric gases can pass and repass without difficulty through the 

 porous shell and its lining membranes. On the commencement of incu- 

 bation, a liquefaction takes place in the albumen immediately above that 

 part of the yolk which is occupied by the blastoderm ; so that the vitel- 

 lus rises toward the surface, by virtue of its specific gravity, and the 

 blastoderm comes to be placed almost immediately underneath the 

 lining membrane of the egg-shell. The body of the embryo is thus 

 placed in the most favorable, position for the reception of warmth and 

 other necessary external influences. The liquefied albumen is absorbed 

 by the vitelline membrane, and the yolk thus becomes larger, softer, and 

 more diffluent than before the commencement of incubation. 



As soon as the circulatory apparatus of the embryo has been fairly 

 formed, two minute arteries are seen to run out from its lateral edges 

 and spread into the neighboring parts of the blastoderm, breaking up 

 into inosculating branches, and covering the adjacent portions of the 

 yolk with a plexus of capillary bloodvessels. The space occupied in 

 the blastoderm by these vessels is called the area vasculosa. The blood 

 is returned from it to the body of the embryo by two veins which pene- 

 trate beneath its edges, one near the head and one near the tail. 



The area vasculosa increases in extent as the development of the 

 embryo proceeds, and its circulation becomes more active. It covers 

 the upper half or hemisphere of the yolk ; and then, passing this point, 



