DEPARTMENT OF EMBRYOLOGY. 73 



DEVELOPMENT OF INDIVIDUAL ORGANS. 

 Vascular System. 



It will be noted that considerable attention has been devoted to the develop- 

 ment of the blood-vessels, and it should be explained that our activities in this 

 field date back to the well-known work of the late Professor Mall. The 

 interest in the problems of blood-vessel formation which he inspired in his 

 coworkers and pupils has led to a series of investigations that have given us a 

 fairly clear conception of the developmental factors that control and modify 

 this important system. While working in Baltimore, Dr. H. M. Evans, by 

 means of his remarkable injection preparations, clearly demonstrated the 

 primitive form of the vessels and the general principle of the manner of their 

 change into the adult pattern. Later, Dr. Sabin, by utihzing the methods 

 of tissue culture, succeeded in demonstrating the cellular origin of endo- 

 thelium. She showed that certain mesenchymal cells are differentiated in 

 situ into angioblasts which have two notable characteristics: (1) as they pro- 

 liferate they adhere together in the form of islands or cords, which tend to 

 unite with other cords, forming a plexus; (2) the central cells of these islands 

 and cords for the greater part liquefy into plasma, whereas the peripheral 

 cells are preserved as endothelium, the essential tissue of blood-vessels. 



Having established the salient features of the differentiation of angioblasts, 



it became important to know how late in embryonic life this phenomenon 



continues. In the chick, Dr. Sabin has been able to demonstrate it for the first 



7 days of incubation, a large part of the dorsal aorta forming in this way. On 



the other hand, in regeneration of blood-vessels in the adult, no evidence of 



differentiation of new angioblasts has ever been found. This brings us to the 



recent observations of Miss Ellen B. Finley, a worker with Dr. Sabin, who 



shows that in the human embryo angioblasts continue to differentiate from 



indifferent mesenchyme as late as the end of the second month of intrauterine 



life. 



Differentiation of Angioblasts in the Human Embryo. 



For determining how late in development of the embryo new angioblasts 

 differentiate. Miss Finley chose, as a favorable place for study, the scalp 

 region of human embryos between 20 and 30 mm. long. At this time the 

 subcutaneous region at the top of the head is avascular and one can study 

 the spreading upward of the growing vascular plexus of the scalp from the 

 sides of the head as it invades the avascular tissues of the vertex. Miss 

 Finley has shown how, in total mounts, the scalp may be spread out as a thin 

 transparent membrane and all parts of the plexus studied under high mag- 

 nifications and with the use of various stains. In this way she has demon- 

 strated histologically the transition from avascular mesenchyme to the 

 characteristic fringe of angioblasts, from this to the primitive capillary net, 

 and from this in turn to the region of well-defined arteries and veins. Such 

 preparations were controlled with sectioned material cut in various planes. 

 Her studies estabhsh the fact that differentiation of angioblasts in the human 

 embryo is actively in process in specimens as large as 30 mm. long. As for 

 the differentiation of the mesenchyme into angioblasts, she finds that it occurs 

 at the tips of the growing plexus, perhaps due to the stimulus of the plexus on 

 the mesenchymal cells lying close to them, and the new angioblasts usually 

 are in direct contact with the tips. Through active proliferation they form 

 large clumps, and almost at once hemoglobin-containing cells can be dis- 



