DEVELOPMENT OF THE CHICK. 603 



placed almost immediately underneath the lining membrane of the 

 egg-shell. As the cicatricula is the spot from which the process of 

 embryonic development commences, the body of the young fcetus 

 is by this arrangement placed in the most favorable position for 

 the reception of warmth and other necessary external influences 

 through the egg-shell. The liquefied albumen is also absorbed by 

 the^itelline membrane, and the vitellus 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 blasto- 

 dermic membrane, breaking up into inosculating branches, and 

 covering the adjacent portions of the vitellus with a plexus of 

 capillary bloodvessels. The space occupied in the blastodermic 

 membrane, on the surface of the vitellus, by these vessels, is called 

 the area vasculosa. (Fig. 211.) It is of a nearly circular shape, 



Fig. 211. 



Eoo OF FOWL during earl7 periods of incubation ; showing the body of the embryo, and the 

 area vasculosa partially covering the surface of the vitellus. 



and is limited, on its outer edge, by a terminal vein or sinus, called 

 the " sinus terminalis." The blood is returned to the body of the 

 foetus by two veins which penetrate beneath its edges, one near the 

 head and one near the tail. 



The area vasculosa tends to increase in extent, as the develop- 

 ment of the fcetus proceeds and its circulation becomes more active. 

 It soon covers the upper half, or hemisphere, of the vitellus, and 

 the terminal sinus then runs like an equator round the middle of 

 the vitelline sphere. As the growth of the vascular plexus con- 



