ORIGIN OF BLOOD AND ENDOTHELIUM 313 



changes into that of an erythroblast. It must be distinctly 

 remembered that these appearances are in dead stained speci- 

 mens and many possibilities exist which might explain their 

 occurrence. 



Granting that such a phenomenon actually appears to occur 

 there is one very probable explanation without assuming that 

 true vascular endothelium may form blood corpuscles. Let it 

 be supposed, for example, that in the formation of the vascular 

 wall around the yolk-sac blood islands that some of the peripheral 

 cells of the island might lag behind in their differentiation re- 

 taining their more or less mesenchymal type. Such a cell may 

 come to be closely pressed against the vascular wall and really 

 appear as though it were one of the vessel wall cells. This 

 might readily happen, and probably does happen, and may ac- 

 count for the occasional appearance of 'vessel wall cell' forming a 

 blood cell. 



Why do not the endothelial cells in the experimental embryos 

 possess the power to form blood cells when the vessel is totally 

 empty of blood cells? Even though it is clearly shown that 

 other cells of the embryo do possess the normal blood building 

 power. These specimens are exactly such as should supply 

 definite proof of blood cells arising from endothelium, but the 

 evidence they furnish really disproves the proposition. 



Schridde ('07, '08) according to Maximow has gone so far as 

 to claim that in young human embryos endothelium can directly 

 form primitive erythroblasts. Maximow does not agree with 

 this since in his specimens the endothelium gives rise only to 

 indifferent colorless cells. Shridde's claim is based upon 

 misinterpretation and so, I believe, is any claim that blood cells 

 arise from formed vascular endothelium. 



Most authors find that in the very early embryonic blood 

 there are no white corpuscles but only red cells present. Bryce 

 ('05) however, describes in Lepidosiren the very early origin of 

 leucoblasts from primitive blood cells, and later Dantschakoff 

 and Maximow find lymphocytes not only in the vascular net of 

 the area vasculosa but also, though at first very few, in the 

 circulating blood. Maximow thinks that when these early 



