160 DEVELOPMENT OF VASCULAR PLEXUS IN SCALP OF HUMAN EMBRYO. 



At the border between the first and second zones are occasionally small clumps 

 of cells which have no visible connections with the main plexus. Plate 2 (figs. 

 12 and 13) shows some of these isolated clumps. They most frequently occur as 

 single chains of nucleated cells containing a slight amount of hemoglobin and often 

 lie in direct fine with the advancing plexus, though not continuous with it. Some- 

 times they are seen as solid clumps of cells, with fine, thread-like processes extending 

 out from them, strongly suggestive of those described by Dr. Sabin in the two-day 

 chick. She found a marked tendency on the part of syncytial masses of angioblasts 

 to put out delicate sprouts by which they joined similar masses, thus developing 

 the vascular plexus. Since most of these isolated chains and clumps of cells contain 

 hemoglobin, they might easily be regarded as indicating the origin of red blood-cells 

 from mesenchyme outside the vascular system, but when their proximity to the 

 main plexus is considered, together with the probability of their joining it to form 

 solid cellular masses, as has been described, their position and their hemoglobin- 

 content do not seem to militate against an angioblastic origin for red blood-cells 

 and endothelium. It seems quite clear that this process is intermediate between 

 the two extreme views. 



There are, it seems, at least three possible explanations for the development 

 of the vascular area in the subcutaneous tissue of the head of the human embryo. 

 First, it is possible to conceive of the tips of the vessels forcing their way into and 

 through the undifferentiated tissue, taking nothing from it, but pushing the mesen- 

 chymal cells aside as they advance by means of their own active cellular division 

 and growth. One would expect, under such conditions, that when the sections of 

 these areas are fixed, the vessels would shrink, leaving in their place a hollow space. 

 This has never been noted, nor have the surrounding mesenchymal cells a com- 

 pressed appearance. A second possibility is that the vessels lengthen by true 

 endothelial division and sprouting. Figures 7 and 8 (plate 2) are suggestive of 

 such a process, but they are the exception rather than the rule, since there appears 

 to be a great enlargement of the vascular tips, due to a marked differentiation of 

 mesenchyme into red blood-cells, before many endothelial cells are clearly dif- 

 ferentiated. Another conception is that the tips of the growing plexus exert just the 

 stimulus needed for the mesenchymal cells lying close to them to differentiate into 

 angioblasts or primitive blood-cells and to become joined to the tips. From obser- 

 vation of many different specimens, the impression has been gained that this last 

 is the principal method of growth. The cells may be added one by one, or they 

 may form single strands before adding themselves to the main plexus. Either before 

 or after becoming a part of the plexus, it is probable that they quickly divide and 

 grow, taking on the appearance of solid masses of cells of varied size and shape. 

 The fact that the mesenchymal cells differentiate in such a precipitous manner 

 into hemoglobin-containing red cells is doubtless to be explained by the relatively 

 late stage of embryonic development at winch the differentiation occurs. 



In closing, I should like to say that this problem was suggested to me by Dr. 

 Sabin, and I am greatly indebted both to her and to Dr. Streeter for helpful advice 

 and assistance throughout the course of the work. 



