254 ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 



specimens in which the matter could be called into question is such a one as that 

 just described, in which there are many cells that I term unicellular angioblasts. 

 These can always be found at the stage of 11 somites in the posterior part of the 

 area pellucida. When such a blastoderm is fixed one might bring up the question 

 as to whether any of the cells outside the vessels might not contain hemoglobin, 

 but I think that the answer is negative, on the ground that at this stage no 

 hemoglobin-bearing cells are forming within the vessels of this area. The question 

 could be definitely settled by more careful records than I have yet made of the 

 evidences of hemoglobin in the living cells. 



In later stages, on the other hand, after a considerable mass of blood has been 

 formed, the presence of true erythroblasts in the interspaces is a very common 

 occurrence in all of the specimens, just as it is a familiar phenomenon in all sections 

 of embryos. I interpret these cells as having escaped from the lumina of the vessels, 

 just as they have been interpreted in the study of sections, and for the same reasons. 

 In the first place, there must be some rupturing of the delicate walls of endothelium 

 in the mounting; often the specimens must be shaken a little in the solution to 

 free them from the vitelline membrane, and it is hardly possible that any artificial 

 medium would not make some change in these thin membranes. Then, just as 

 Madame Danchakoff (1909, p. 125) finds in sections, I find in the living specimens 

 that these cells tend to degenerate. Moreover, I have never observed them to move 

 toward the vessels, indicating that, like the erythroblasts within the vessels, they 

 have but little motility. The question as to whether any red cells of the chick ever 

 differentiate outside the lumen of a vessel is one of very considerable importance on 

 account of the theory that they do so differentiate in mammals. So far the evidence 

 in the study of these living forms is that in the chick erythroblasts differentiate 

 only within the lumen of a vessel. I have made but few pecimens of 3 or 4 

 days of incubation. The method can be applied up to the fourth day, and in one 

 such specimen there is an isolated vesicle near one of the branches of the artery 

 which is packed with erythroblasts, showing that the formation of new angioblasts 

 giving rise to vesicles is going on, and again emphasizing the intravascular forma- 

 tion of the red cells. 



Another point of significance, which these observations on the living specimens 

 seems to me to settle definitely, is that all of the cells of the blood-islands of the 

 chick during the first two days of incubation become red cells; that is, they all 

 develop hemoglobin. This is very striking in the living chick in which every single 

 cell of an island can be seen to be yellow, and all of the cells of a given island are 

 uniformly yellow. In fact, all of the cells of these islands appear alike except for an 

 occasional dead cell among them. These, as has been said, react to neutral red, 

 and in fixed preparations show picnotic and fragmented nuclei. Thus the only cells 

 which by any chance could be confused with white blood-cells can be proved to be 

 dead cells. The granular corpuscles can be identified in the blood-vessels in the 

 living chick at the stage of the fourth day of incubation and their origin must 

 therefore be taken up in later stages. 



