STUDIES ON THE ORIGIN OF BLOOD-VESSELS AND OF EED 



BLOOD-CORPUSCLES AS SEEN IN THE LIVING BLASTODERM 



OF CHICKS DURING THE SECOND DAY OF INCUBATION, 



BY FLORENCE R. SABIN. 



INTRODUCTION. 



In the study of the origin of endothelium and blood-cells it is obvious that any 

 extension of the method of observing them in living form is an advance. Until 

 recently the tadpole's tail constituted the main subject for the study of growth of 

 the vascular system in a living form. From the time of the discovery by Kolliker 

 that it contains two kinds of capillaries, those carrying blood and those carrying 

 lymph, the tadpole's tail has been studied over and over again in connection with 

 the question of the growth of vessels. The final studies on the living tadpole, made 

 by E. R. Clark (1909, 1918), have demonstrated conclusively that at certain stages 

 both blood-capillaries and lymph-capillaries in the area of the tail grow by the 

 method of sprouting; that is, by the division of and increase in the cytoplasm of the 

 endothelium lining the vessels, and not by the addition of new mesenchymal cells 

 to their walls. 



An extension of the study of the vascular system to a living form was then 

 made by Stockard (1915), who watched blood-vessels arise in developing fish 

 embryos. This author showed that the first sign of blood-vessels is the elongation 

 of certain mesenchymal cells into spindle-shaped cells which could be identified as 

 angioblasts. This I believe to be a fundamental point that the vascular system 

 begins by the differentiation of certain cells which we may term angioblasts, or 

 according to Ranvier, vasoformative cells. Stockard then subjected the fish to 

 experimental conditions, by which he made an analysis of the place of origin of 

 red blood-cells. In fish embryos that had been treated by certain chemicals he 

 produced an abnormality consisting of a lack of union between the venous end of 

 the heart and the vitelline veins, so that there was no circulation and consequently 

 no moving of the blood from place to place. From these experiments he concluded 

 that in the fish all of the cells that form red blood-cells are located in the 

 intermediate cell-mass in the posterior part of the embryo, and that none of them 

 arise in the head. In this conclusion his results have been questioned by Reagan 

 (1917) in a study on the same form. Certainly in the chick it can be definitely 

 proved that the endothelium of the vessels gives rise to red blood-cells, and that 

 this process can be seen to take place not only in every part of the area vasculosa, 

 but within the embryo itself, the process having been observed in the aorta in the 

 living chick. 



Observations on blood-vessels in living chicks are by no means new; in fact 

 it is obvious that many of the conclusions in the monograph of His, published in 



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