86 THE THIKD DAY. [CHAP. 



The blood leaving the body of the embryo by the omphalo- 

 mesaraic arteries (Fig. 23, R. Of.A., L. Of. A.), is carried to 

 the small vessels and capillaries of the vascular area, a small 

 portion only being appropriated by the pellucid area. 



From the vascular area part of the blood returns directly 

 to the heart by the main lateral trunks of the omphalo- 

 mesaraic veins, R. Of., L. Of. During the second day these 

 venous trunks joined the body of the embryo considerably 

 in front of, that is, nearer the head than, the corresponding 

 arterial ones. Towards the end of the third day, owing to the 

 continued lengthening of the heart, the veins and arteries 

 run not only parallel to each other, but almost in the same 

 line, the points at which they respectively join and leave 

 the body being nearly at the same distance from the head. 

 According to Von Baer and other observers the veins in the 

 vascular area are placed nearer the surface thau are the 

 arteries. Close to the body the reverse is the case ; near the 

 body therefore they cross over each other. 



The rest of the blood brought by the omphalo-mesaraic 

 arteries finds its way into the lateral portions of the sinus 

 terminalis, S.T., and there divides on each side into two 

 streams. Of these, the two which, one on either side, flow 

 backward, meet at a point about opposite to the tail of the 

 embryo, and are conveyed along a distinct vein which, run- 

 ning straight forward parallel to the axis of the embryo, 

 empties itself into the left omphalo-mesaraic vein. The 

 two forward streams reaching the gap in the front part of 

 the sinus terminalis fall into either one, or in some cases 

 two veins, which run straight backward parallel to the axis 

 of the embryo, and so reach the roots of the heart. When 

 one such vein only is present it joins the left omphalo- 

 mesaraic trunk ; where there are two they join the left and 

 right omphalo-mesaraic trunks respectively. The left vein is 

 always considerably larger than the right; and the latter 

 when present rapidly gets smaller and speedily disappears. 



The chief differences then between the peripheral cir- 

 culation of the second and of the third day are due to the 

 greater prominence of the sinus terminalis and the more 

 complete arrangements for returning the blood from it to the 

 heart. After this day, although the vascular area will go on 

 increasing in size until it finally all but encompasses the 



