256 ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 



CONCLUSIONS. 



These studies have shown that throughout the first two days of incubation there 

 is a continual differentiation of mesenchymal cells into angioblasts. In the chick these 

 cells arise first in the embryonic membranes, but by the stage of 5 somites they begin 

 to differentiate actively in the embryo itself and both the endocardium and aorta 

 can be seen to differentiate in situ. Moreover, angioblasts continue to differentiate 

 in the wall of the yolk-sac during the third and fourth days of incubation. 



The characteristics of these new angioblasts have been made sufficiently sharp 

 so that they can be identified in sections. In this way we can be sure that not only 

 the aorta but the primitive vessels of the embryo, such as the vessels along the 

 nervous system and the cardinal veins, differentiate in situ. These studies must 

 leave open the question as to the time, if ever, when angioblasts cease entirely to 

 differentiate and all of the growth comes to be from the walls of previously formed 

 vessels, as was observed by Clark (1909) for the tadpole's tail. The place at which 

 to attack this problem again seems to me to be in further studies on the regeneration 

 of vessels in healing wounds. 



In these observations on the living blastoderm of the chick it has been shown 

 that it is possible to make a sharper distinction between the cells which differentiate 

 to form the ccelom and those which form the vascular system. It therefore seems 

 better to use the term primitive mesoderm for the masses of cells which will give rise 

 to both structures. Indeed, the relationships can be made quite clear by Limiting 

 the term blood-island to those masses of cells that actually develop hemoglobin and 

 become erythroblasts, while the indifferent masses, which will give rise to mesoderm 

 on the one hand and angioblasts on the other, are given a less specific name. 



The exocoelom forms by a splitting apart of the two layers of cells which come 

 from the primitive mesoderm. Blood-vessels, on the other hand, arise by the 

 differentiation of a new type of cell from this same primitive mesoderm. This has 

 a different cytoplasm from the original mesodermal cell, is more granular, more 

 basophilic, and has new qualities, namely, the tendency to form solid masses which 

 appear like a syncytium, the centers of 



Which liquefy tO form blood-plasma, and a ^differentiated mesenchyme cell 



marked tendency to put out delicate 



SprOUtS by Which it joins similar masses. 



Thesecells,orangioblasts,giverisetoendo- _ 



,iv 11 i-i i i i_i 11 Endothelium Blood-island(erythroblast) 



thehum, blood-islands, and blood-plasma. 

 All of the cells of the blood-islands of the first two days of incubation become 

 erythroblasts. The lumina of the vessels is not formed of tissue-spaces, but rather 

 by a process of cytolysis in the center of masses of cells or even within the cyto- 

 plasm of a single cell. An endothelial cell differentiates in its turn from the original 

 angioblast. It can give rise to other endothelial cells or to erythryoblasts. 



The observations herein recorded do not bear on the question of the origin of 

 white blood-cells, because there are no cells in the chick of the second day of incuba- 

 tion that can be identified as the ancestors of the white cell. They do show, however, 

 that the ancestry of the red cells can be outlined as shown by the diagram above. 



