240 ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 



the younger and older cells of a given specimen vary in their reaction to a given 

 concentration of salt; but that the erythroblasts as a whole are characterized by 

 their very slight power of amoeboid movement. Their contours change slightly, 

 as seen in a living specimen, but the cells themselves move from place to place 

 exceedingly slowly when not swept along in a current of fluid. They are elastic 

 but not very motile. 



In regard to the breaking up of the blood-islands, the only process which I have 

 actually seen in the living chick is the slow freeing of individual cells from the edge 

 of the mass, but so many fixed specimens look as if islands had been caught just as 

 all the cells were going to break apart at once, that I can readily believe this does 

 actually take place. 



We have thus described the processes by which two different structures develop 

 out of the primitive mesoderm, the ccelom on the one hand and the blood-vessels 

 on the other. We have shown that blood-vessels arise from cells called angioblasts, 

 which differentiate from mesoderm and produce endothelium, blood-plasma, and 

 red blood-corpuscles. We have emphasized the importance of the destruction of 

 cells in the production of the first blood-plasma and have shown that this plasma 

 is different in origin from the tissue-fluid. Moreover, it has become clear that it is 

 inadvisable to identify as blood-islands the primitive masses of mesoderm which 

 are to give rise to both of these structures, for the reason that they must first split 

 into cells which will form two layers for the coelom and those which will develop 

 a different type of cytoplasm and form clumps of cells, the forerunners of vessels. 

 Besides this, these original cells are not all the forerunners of blood-cells, but rather 

 are masses which are to be further differentiated into those which form endothelium, 

 with the potentiality of producing cells which can themselves make hemoglobin 

 and those that become blood-islands. Since one can now distinguish all these types 

 of cells, a more restricted terminology would seem to be of value. That is to say, 

 the original masses of cells in the blastoderm, long known as blood-islands, we 

 might call by the general term primitive mesoderm, and distinguish three types of 

 cells, i. e., (1) angioblasts; (2) the cells forming blood-islands (cells anchored to the 

 endothelial lining of vessels, which develop hemoglobin in their mass and which are 

 derived either directly from angioblasts or from endothelium); and (3) primitive 

 erythroblasts or cells which have become free from the islands but which go on 

 dividing actively within the lumen of the vessels. 



In his studies on living fish embryos Stockard (1915) did not find evidence that 

 endothelium can produce blood-cells in that form. He says (p. 229) : 



"There are numerous descriptions and illustrations of the origin of blood-cells from 

 the vessel linings in the literature of the last twenty-five years, since Schmidt in 1892 

 described the transformation of individual endothelial cells into white and red blood- 

 corpuscles. Yet again, I believe that the really skeptical reader will not be at all convinced 

 that such a thing really ever takes place, from the evidence presented in the literature, 

 certainly not from any of the illustrations that have been made of this process. No real 

 vascular endothelial cell has ever been actually observed to metamorphose into a blood-cell, 

 or to divide off another cell which forms a blood-cell, and until such a direct observation is 

 forthcoming one can only question the accuracy of the interpretation of the various obser- 

 vations up to now recorded." 



