DEVELOPMENT OF WANDERING MESENCHYMAL CELLS 553 



from their long sides. In a 48 hour embryo these cells already 

 present the appearance of individual endothelial cells. They 

 migrate indefinitely for a few hours and then tend to group them- 

 selves in more or less irregular collections. 



Up to this time no one from mere observation could be abso- 

 lutely certain that the cells of this rather characteristic ap- 

 pearance are actually to become vascular endothelial cells in all 

 cases. The possibility, of course, exists that the elongate spindle 

 cells may at times round up and then assume the more amoeboid 

 shape of the probable future chromatophore. Yet since the 

 shape of these cells is so characteristic and such shapes are so 

 constantly present, one is inclined to believe that the same cell 

 may actually retain this character until it really becomes a com- 

 ponent part of a vascular endothelial arrangement. 



Figures 19, 20 and 21 illustrate the region along the side of the 

 embryo's head at 48 hours old. It is in this region that the first 

 large yolk vessel develops. This vessel carries blood from the 

 body of the embryo around a short circuit to reach the venous 

 end of the heart and thus in a way relieves the flow that other- 

 wise would force itself through the small poorly formed vessels 

 in the embryonic body. This vessel is, therefore, of necessity 

 one of the earliest to develop. The three figures, 19, 20 and 21, 

 show variations in the arrangement of the wandering mesen- 

 chymal cells in the region of the future vessel. 



In figure 19 there is really no definite cell aggregation except 

 along the edges of the head mesoblast as it spreads somewhat 

 over the yolk, yet a few of the cells show the typical spindle 

 shape. Figure 20 indicates a tendency of the mesenchymal 

 cells to line themselves in a group exactly along the course of the 

 coming vessel. Many of the cells in this group give the actual 

 appearance of an endothelial cell after it is fully developed and 

 forming one of the units in a vessel wall. The embryo illustrated 

 by figure 21 shows much the same condition. Very few mesen- 

 chymal cells occur between this cell aggregation and the side 

 wall of the head. Lateral of the vessel group the cells are also 

 not numerous and have no system of arrangement. 



