DIRECT GROWTH OF VEINS BY SPROUTING. 7 



of the center of a solid mass ofeells, so that they form, not secondary to a collection 

 of fluid in mesenchymal spaces, but by a transformation of mesenchyme cells into 

 angioblasts which then produce both the fluid and the endothelial boundary. 



It is interesting to note that all of the facts brought forward by Kampmeier 

 (1922), in his recent restudy of the origin of the lymphatic system in amphibia, 

 are virtually an account of the origin of the lymphatic system by the differentiation 

 of angioblasts, their transformation into vessels, and their uniting to make lym- 

 phatic plexuses. When the subject is restudied, it will be found, I am sure, that 

 the same sequence of events can be demonstrated in any of the zones in which 

 lymphatics are differentiating; that is to say, the fundamental principles of the 

 origin of the entire vascular system, including lymphatics, are known. It is, of 

 course, clear that we are as far as ever from analyzing the cause of this differentia- 

 tion and are stating merely a sequence of events, that the cell precedes the forma- 

 tion of the fluid of the blood or of the lymph rather than that fluid collects and 

 causes a flattening out of cells to line a space. If the third hypothesis of Thoma, 

 namely, that in the spread of vessels into organs it is, in the last analysis, the 

 organs themselves that determine vessels, proves to be the most fundamental law 

 in connection with the growth of the vascular system, certain factors in the environ- 

 ment of developing vessels are not beyond the range of experimentation. Indeed, 

 such studies have already been started by Stockard (1915) and, if carried farther, 

 might throw great light on the extent to which the vascular system is determined by 

 its environment. 



In the early studies of the spread of vessels over the embryo, as developed 

 by the method of injection, there grew up the theory that the growth of vessels 

 is wholly within the capillary bed. This was a natural deduction from the fact 

 that during the stages in which vessels are spreading over the embryo the wall of 

 the vessel is almost everywhere limited to a lining of endothelium, so that the idea 

 was correlated with the fact that the entire vascular system started on the basis 

 of the structure of the capillary. In fact, the aorta begins as a vessel with a lining 

 of endothelium only and remains without either muscle or adventitia for a long 

 time after the circulation has begun. Indeed, the heart is the only part of the 

 vascular system in which the musculature begins to differentiate at the same time 

 the endothelial lining is itself forming from angioblasts. It appears, then, that 

 in the spreading of the vascular system the capillary plexus precedes the artery 

 and vein. There are, however, exceptions to the general rule that each vessel comes 

 from a preliminary plexus, since the aorta itself, certainly in a part of its course, 

 forms from chains of angioblasts rather than from any very complicated plexus. 



In the present paper are presented certain observations concerning the growth 

 of veins, which have a bearing on these fundamental relations. In the study of 

 the vessels in the area vasculosa of the living chick it has been found possible to 

 make preparations of the area pellucida throughout the period of incubation. 

 The embryo itself can be kept attached only through the early part of the fourth 

 day, because it then becomes too heavy to remain against the cover-slip in the 

 reversed position of the hanging drop preparation, and as it sags away from the 



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