158 DEVELOPMENT OF VASCULAR PLEXUS IN SCALP OF HUMAN EMBRYO. 



portion of the vascular spread with the vascular elements intact and with their 

 normal relations preserved, in contrast with serial sections in which the fields 

 were necessarily discontinuous and comparatively limited. 



In the gross examination of human embryos ranging from 19 to 45 mm., there 

 could be observed a delicate, fringe-like plexus pushing up toward the vertex of the 

 head, first described by Hochstetter (1916). It was always visible, but in some 

 embryos it had a much more brilliant appearance than in others, due, possibly, 

 either to the stage of development of the plexus at that particular time or to the 

 fixing fluid used. It was most striking in an embryo that had been fixed in Bouin's 

 solution, which is probably much better for this purpose than formalin. At 

 earlier stages the transition from vascular to avascular conditions is more gradual. 

 In later stages it is more abrupt and the transitional margin is in the nature of a 

 narrow, well-defined line. The early stage is particularly well shown in plate 1, 

 figure 2, where there appear to be two prominent foci of growth or growth centers, 

 one anterior to the ear in the temporo-frontal region and the other posterior to the 

 ear in the occipital region. The growth of the vessels radiates up and out from 

 these centers. On the borders of these two semicircular areas, small, finely granular 

 tips can be seen, a few of which seem to have no connection with the larger vessels 

 below. The growing edge, as it advances, tends to become more and more flattened, 

 as may be seen in figures 3 and 4, a slight indentation, however, persisting at a point 

 almost directly above the anterior portion of the external ear. At about 40 to 

 45 mm. the growing tips anastomose across the mid-line and circulation over the 

 head is established. 



On microscopic examination of total mounts from the head region, four stages 

 in the development of the blood-vessels were observed. Figure 1 shows diagram- 

 matically four definite zones. First, toward the vertex, is the uppermost zone, 

 which is a predominantly avascular area composed of undifferentiated mesenchyme. 

 The zone next below consists of a network of solid, darkly staining masses of nucle- 

 ated cells filled with hemoglobin. Toward their upper borders these masses often 

 have slender tips which penetrate the avascular area. Between and beyond the 

 tips stretches indifferent mesenchyme. This second zone may be called the zone 

 of the angioblastic net. The third zone is a capillary network and in it can be seen 

 delicate, branching capillaries whose endothelial walls appear to be intact and to 

 inclose a definite lumen. Within the lumina of these vessels are scattered clumps 

 of well-formed blood-cells (nucleated and non-nucleated), whose outlines are 

 clearly defined. Occasionally the lumen is practically empty (plate 2, fig. 10), 

 the most probable explanation for which is that liquefaction of cellular elements 

 has taken place within the blood-vessels themselves, assuming that this area has 

 been transformed from the solid zone just described. Finally, in the last zone 

 are encountered more mature vessels, with slightly thickened walls, through which 

 blood has evidently circulated to some extent. Some of these vessels may be 

 forerunners of vessels destined to persist. 



These zones are the expression of a developmental process, and in the growing 

 state the characteristic elements of one zone must become quickly transformed into 



