64 DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 



The second question which has proved of great interest is the definition of 

 the terms artery, vein, and capillary as they are used for the embryo. In the study 

 of the vessels of the embryo particular stress should be laid on the time when circu- 

 lation begins. That there is a very extensive development of blood-vessels before 

 there is any circulation of the blood due to the beat of the heart was well known to 

 the earlier embryologists; for example, to von Baer and later to His. Moreover, 

 the heart beats for a considerable time before it starts any circulation. It is known 

 that the blood-vessels spread over the body in definite and constant sheets of capil- 

 laries, and in these primitive vessels, after the circulation has begun, a vessel may 

 serve as an artery for a time and then be reduced to a capillary plexus, in which the 

 direction of the circulation is entirely different from that of the circulation of the 

 original artery. Such a vessel, for example, is the subintestinal artery of the pig, 

 which arises in a capillary plexus around the caudal end of the primitive gut and 

 carries blood out to the arteries of the yolk-sac, where it must again pass through a 

 capillary bed before it returns to the heart. This artery becomes broken into a 

 capillary plexus in the wall of the gut, which makes new connections with branches 

 of the omphalo-mesenteric veins within the wall of the mesentery, so that its blood, 

 instead of flowing away from the embryo to the membranes, flows within the embryo 

 toward the heart. 



Again, a vessel may serve for a time as a vein in the return of blood to the heart 

 and may subsequently receive new arterial connections and become an arterial 

 plexus, with the direction of the flow of blood entirely changed. Such a vessel is 

 the so-called vena capitis medialis. This is a primitive vessel along the hind- 

 brain, which in the chick in the second and third days of incubation serves as a vein 

 for the forebrain and midbrain, but as an arterial capillary trunk for the hind- 

 brain; that is, it carries mixed blood and is the only vessel of the hindbrain, 

 representing its entire capillary bed. Early in the fourth day it receives new 

 arterial connections, a new vein develops to carry the venous blood for the fore- 

 brain and midbrain, and the primitive vascular channel of the hindbrain breaks 

 into a capillary plexus in which the direction of the current of blood is at right angles 

 to the direction of the original current. From these two examples it must be clear 

 that in the study of the primitive vascular system it is very important to understand 

 the function of the vessels at each stage of development, and any presentation of the 

 vascular system which overlooks this point and is dominated wholly by the pattern 

 of the vessels of the adult becomes difficult to follow and may be misleading. In 

 the question of nomenclature a decision has to be made between two theories that 

 is, whether the vessels are to be named according to the function they perform at 

 any given stage or whether they are to be named according to the vessels for which 

 they form the primordia. If the latter method is chosen it must be remembered 

 that a given vessel of an embryo often disappears entirely in giving rise to new ves- 

 sels- for example, the primitive vessel of the hindbrain. 



In this study I shall use terms as consistently as possible, in the following 

 manner: By the term artery, in reference to an organ, I mean a vessel which brings 

 blood to that organ but does not form any part of its capillary bed, and I have 

 colored such vessels red. By the term vein I mean a vessel which carries blood from 



