ORIGIN AND DEVELOPMENT OF THE PRIMITIVE VESSELS OF THE 



CHICK AND OF THE PKi. 



BY FLORENCE R. SABIN. 



INTRODUCTION. 



In this paper is given an account of the primitive vessels of the chick and of 

 the pig, as made out by injecting living embryos, and, in the case of the chick, as seen 

 growing in the embryo. Such studies must necessarily be accompanied by the 

 study of sections. In the case of the mammalian embryo I have made injections in 

 earlier stages than had been done heretofore and in the case of the chick I have 

 carried the method of injection to the earliest stage in which it is possible. Below 

 the stage at which they can be injected, the vessels of the chick can be studied in 

 the living blastoderm by a technique which has developed out of the method of 

 tissue-culture introduced by Harrison. The chick thus offers unusually valuable 

 material for the study of vascular problems, as it is possible to use both the method 

 of injection and that of direct observation of the living embryo in the same stage. 



In the course of this study two fundamental ideas have been under considera- 

 tion. The first concerns the most essential question in connection with the vascu- 

 lar system, namely, the relation of differentiation and growth of endothelium. 

 According to one theory there is a limited period for the differentiation of angio- 

 blasts out of undifferentiated mesenchyme, and after this period all new blood-vessels 

 arise from the growth or proliferation of older angioblasts. This theory seems to 

 me to have the weight of evidence. The second theory is that angioblasts continue 

 to differentiate out of mesenchyme indefinitely. If the former theory is correct and 

 the period of differentiation of endothelium is a limited one, the fundamental prob- 

 lem concerning the early blood-vessels is to determine which differentiate and which 

 are formed from preceding vessels. In practically every embryo chick observed, 

 up to a certain stage new angioblasts can be seen differentiating and joining the 

 older angioblasts, but the phenomenon becomes less and less frequent as older 

 stages are studied. In the living embryo the aorta itself can be seen to differentiate 

 out of mesenchyme, and at the stage when the heart begins to beat every chick 

 shows a few isolated angioblasts along the mesial border of the aorta, which will 

 be seen to join the aorta if the specimen be watched for a short time. I have some 

 evidence also that some of the primitive vessels along the neural tube differentiate 

 out of mesenchyme, the process being observed in the living embryo. On the other 

 hand, one can watch the growth of the entire wall of a vessel by cell-division in the 

 living embryo and the formation of new vessels from the walls of old vessels; so that 

 the study of the early blood-vessels is gradually becoming a more exact problem, 

 namely, the determination for each vessel, whether it differentiates in situ or develops 

 from preceding vessels. My present material is not adequate for the solution of 

 this question, but throws some light upon it. 



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