120 DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 



are intersegmental branches of the cardinal veins. It is thus clear that the general 

 direction of the blood to the neural tube is from the ventral to the dorsal border 

 and that the direction of the flow of blood from the neural tube is the reverse. 



In connection with the pig it has been shown that the large branches of the 

 aorta near the caudal end of the embryo are primary allantoic arteries which run 

 in the splanchnopleure, and that the umbilical arteries in the somatopleure 

 develop later and anastomose with the primary allantoic arteries, exactly as in 

 the chick. I have also given an analysis of the subintestinal vein of the chick 

 and of the corresponding artery in the pig, and have shown that the fact that the 

 vessel is an artery in the pig means that the primitive type of circulation of the 

 yolk-sac persists longer in that form than in the chick. It has been shown that 

 both the primitive allantoic arteries and the subintestinal arteries arise in a capil- 

 lary plexus and end in a capillary plexus, so that in the case of these two vessels 

 the blood must pass through two capillary plexuses in its return to the heart. 



The study of the circulation of early embryos by means of injecting living 

 embryos and watching the flow of the ink in them or by watching the circulation 

 of the blood in the living specimen brings out some remarkable changes in the 

 direction of the circulation; for example, the change in the direction of the cir- 

 culation in the vessels of the area vasculosa in the chick when the veins invade a 

 plexus which had been arterial. Again, in connection with the development of 

 the primitive vessel of the hindbrain into a capillary plexus, the direction of the 

 circulation is entirely changed. In the original vessel the blood flowed from the 

 cephalic to the caudal border of the hindbrain, while when the new arterial connec- 

 tions bring blood to the entire ventral border of the vein the blood begins to flow 

 from the ventral to the dorsal border of the hindbrain. In the case of the subin- 

 testinal artery is a third example of a profound change in the direction of the 

 circulation. The blood originally runs through this artery out to the yolk-sac, but 

 when the vessel becomes a capillary plexus in the wall of the gut, the blood flows 

 toward the heart within the embryo in the new mesenteric veins. 



From these studies it is clear that it is important to consider each vessel of 

 the embryo from the standpoint of the function it performs throughout its develop- 

 ment and that the effort toward a precise usage of the terms artery, capillary 

 plexus, and especially of the term vein, is an effort to understand the circulation 

 of the embryo. 



