666 DEVELOPMENT OF THE CIRCULATORY APPARATUS. 



newly-formed tissues. In the egg of the fish (Fig. 253), the princi- 

 pal vein is seen passing up in front underneath the head ; while the 

 arteries emerge all along the lateral edges of the body. The entire 



Fig. 252. 



Fig. 253. 



EGO OF FOWL in process of development, showing area vasculosa, with vitelline circulation, 

 terminal sinus, &c. 



vitellus, in this way, becomes covered with an abundant vascular 

 network, connected with the internal circulation of the foetus by 

 arteries and veins. 



Very soon, as the embryo and the entire egg increase in size, 

 there are two arteries and two veins which become 

 larger than the others, and which subsequently 

 do the whole work of conveying the blood of 

 the foetus to and from the area vasculosa. These 

 two arteries emerge from the lateral edges of 

 the foetus, on the right and left sides ; while the 

 two veins re-enter at about the same point, and 

 nearly parallel with them. These four vessels are 

 then termed the omphalo-mesenteric arteries and 

 veins. 



The arrangement of the circulatory apparatus 

 in the interior of the body of the foetus, at this time, is as follows : 

 The heart is situated at the median line, just beneath the head and 

 in front of the oesophagus. It receives at its lower extremity the 

 trunks of the two omphalo-mesenteric veins, and at its upper 

 extremity divides into two vessels, which, arching over backward, 

 attain the anterior surface of the vertebral column, and then run 

 from above downward along the spine, quite to the posterior 



EGG OF FISH (Jar 

 rabacca), showing vitel 

 line circulation. 



