1 64 Vertebrate Embryology 



an aeration of the blood to take place, and 

 the vascular area serves both as an organ of 

 absorption and of respiration. In fact the 

 vascular area, at this time, reaches its greatest 

 activity, for while it may become greater in 

 extent, it later loses the function of a respira- 

 tory organ (after the formation of the allan- 

 tois), and serves, for the rest of the period of 

 incubation, merely as an organ of absorption. 

 There are certain changes that take place in 

 the vascular area during the third day. Owing 

 to the growth of the embryo, the vitelline 

 veins, which, during the second day, were some 

 distance in front of the vitelline arteries, are 

 brought nearer and nearer to these arteries un- 

 til they lie so close to them that the two ves- 

 sels can hardly be distinguished. During this 

 day the sinus terminalis reaches its greatest 

 functional activity. The blood that empties 

 into it from the vitelline arteries flows, as be- 

 fore, both forwards and backwards, that is, 

 towards the head and towards the tail. The 

 blood that flows towards the head usually gets 

 back into the vitelline veins through two large 

 vessels that lie parallel to the long axis of the 

 embryo (Fig. 56) ; occasionally, however, there 

 is only one of these vessels, the one which 



