134 



CHARLES R. STOCKARD 



oCi O o</ 



Figs. 19, 20 and 21 Outlines of the head regions of three living embryos from 

 48 to 50 hours old, showing different conditions in the grouping of mesenchymal 

 cells on the yolk which later give rise to the large vessel that short circuits blood 

 from the side of the embryo around over the yolk to the venous end of the heart. 

 The future vessel wall is now separate spindle shaped mesenchyme cells. 



This cellular aggregation may then be regarded as the actual 

 anlage of the vascular endothelium of the future vessel. The anlage 

 consists merely of a group of separate wandering mesenchyme cells, 

 and not of a capillary net in any sense. 



