DEPARTMENT OF EMBRYOLOGY. 97 



feature becomes less conspicuous, but in the last analysis the adult 

 conditions are exactly the same and the individual cells live in the same 

 primitive way in the tissue fluids which bathe them. 



INDIVIDUAL SYSTEMS. 



The study of Professor F. R. Sabin, on the origin of blood-vessels 

 in the blastoderm of the chick, which was referred to in the report of 

 this department for the year 1918, has after further extension been 

 published in its final form. By the application of the tissue-culture 

 methods of W. H. and M. R. Lems, Dr. Sabin found it possible to make 

 observations on living blastoderms and was able to trace the process 

 of development of the vascular system back to its very beginning. 

 From such preparations one can clearly demonstrate that throughout 

 the first two days of incubation there is a continual differentiation of 

 primitive mesenchymal cells into angioblasts, the forerunners of endo- 

 thelium and red blood-cells. The process begins in the embryonic 

 membranes, but by the stage of five somites the differentiation is in 

 active process in the embryo itself, both the endocardium and the aorta 

 differentiating in situ. In the wall of the yolk sac the differentiation 

 is still in progress during the third and fourth days of incubation. How 

 long the process continues and when the growth becomes restricted 

 to the walls of previously formed vessels are still open questions. 

 Dr. Sabin is attempting at the present time to throw light on this point 

 by a study of the regeneration of vessels in healing wounds. 



The process of differentiation of the primitive mesenchyme cells is 

 inaugurated by a mottling of the membranes, especially of the pos- 

 terior half. The mottled areas were called "blood islands" by the 

 earlier embryologists, but Dr. Sabin's observations show that these 

 masses form not only the vascular system but the coelom as well. 

 The term blood island is therefore restricted by her to the clumps of 

 cells that actually develop hemoglobin and become erythrocytes. The 

 blood islands are derived either directly from angioblasts or from the 

 endothelial walls of the early vessels. 



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

 derived from the primitive mesoderm. Blood-vessels, on the other 

 hand, arise by a differentiation of angioblasts from this same primitive 

 mesoderm. They form as solid syncytial masses or plexiform cords in 

 which lumina are subsequently produced by a liquefaction of the 

 cytoplasm and nuclei of some of the component cells. The angio- 

 blasts thus give rise to endothelium, red blood cells, and blood plasma. 

 The peripheral portion of the angioblasts that form endothelium gives 

 rise to other endothelial cells or to erythroblasts. These processes 

 begin in the area opaca and gradually extend over the area pellucida 

 to the embryo itself, each new mass of angioblasts differentiating 

 in situ and joining the masses already formed. While these processes 



