DEVELOPMENT OF VASCULAR PLEXUS IN SCALP OF HUMAN EMBRYO. 



159 



Avascular 



Angioblastic 

 plexus 



Capillary 

 plexus 



those of the more mature zone adjoining it. Thus, in any given preparation there 

 is a consecutive picture of the life history of a blood-vessel, from the earliest stage 

 to maturity, from undifferentiated mesenchyme, 

 through angioblast and capillary, to a fully formed 

 vessel. 



The second zone that of the angioblastic 

 net is particularly interesting, not only because 

 it represents the area of actual new growth, but 

 also because of its possible significance in connec- 

 tion with the relation of red blood-cells to endo- 

 thelium. Plate 2 (figs. 7, 8, 9, and 11) shows a few 

 of the varied forms which this plexus assumes. 

 Some of the tips are club-shaped, some thick at 

 the center with two side extensions, like tiny 

 branches on a tree, some so vaguely outlined dis- 

 tally as to seem to merge directly into the mesen- 

 chyme of the avascular area, while others, slender 

 and long, are drawn out into a fine filamentous 

 point. The cells of this zone are all nucleated and, 

 for the most part, contain a considerable amount 

 of hemoglobin. Those at the extreme tips con- 

 tain less, while in a few cells the cytoplasm is en- 

 tirely colorless and translucent (fig. 9). The cell 

 boundaries are not clear-cut, and the cells vary 

 greatly in shape and size, due to their pressure 

 against each other. In this area there are indica- 

 tions of a very massive transformation of mesen- 

 chyme into red blood-cells. In occasional in- 

 stances the cellular masses are edged by long 

 endothelial cells, but for the most part they are 

 entirely composed of the earliest forms of red 

 blood-cells, then rounded contours marking the 

 boundaries between the angioblastic net and the 

 avascular zone. It is obvious that this is not 

 exactly the process by which it has been dem- 

 onstrated that red blood-cells arise in the chick, 

 because it can not be stated that these cells orig- 

 inated within the lumen of a vessel (Danchakoff, 1908; Sabin, 1920). On the other 

 hand, it can not be said that these observations indicate a diffuse extravascular origin 

 of red blood-cells that would subsequently have to migrate into preformed vessels, 

 such as Maximow (1909) believes characterize the late origin of red blood-cells 

 in the mammal. Rather, the process seems somewhat intermediate between these 

 two positions, the cells clearly arising in a definite relation to the vascular system, 

 not quite independently. 



Definitive 

 vessels 



Text-figure 1. 



Diagrammatic sketch of the growing edge 

 of the subcutaneous plexus in the head of the 

 human embryo, showing the four zones of 

 transition from undifferentiated mesenchyme 

 into definite vessels. Processes from the 

 angioblastic plexus can be seen encroaching 

 upon the territory of the avascular zone; these 

 are indicated by lighter shading. 



