112 THE THIRD DAY. [CHAP, 



outline and the arteries are made black. The whole blasto- 

 derm has been removed from the egg and is supposed to be 

 viewed from below. Hence the left is seen on the right, and 



vice ' 



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. 



The rest of the blood brought by the vitelline 

 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 

 each side, flow backward, meet at a point about oppo- 

 site to the tail of the embryo, and are conveyed along a 

 distinct vein which, running straight forward parallel to 

 the axis of the embryo, empties itself into the left vitel- 

 line 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 vitelline trunk; where there 

 are two they join the left and right vitelline 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 

 circulation 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 vas- 

 cular area will go on increasing in size until it finally 



