ORIGIN OF BLOOD AND ENDOTHELIUM 59 



blasts before the existence of a common mother cell is proven. 

 This very necessary observation has never been made and all 

 of the evidence in the present literature seems insufficient to 

 warrant the conclusion that such a thing does actually take place, 

 whereas there is considerable evidence to indicate that white 

 and red blood cells probably arise from two different mesenchymal 

 cells. Of course, these two parent mesenchymal cells may be, 

 so far as our powers of observation go, indistinguishable. Yet 

 this would not indicate that they were not different in their 

 potentialities. One of the two mesenchymal cells might be 

 capable of giving rise to the various types of white blood cells 

 depending upon the conditions of differentiation and function, 

 while the other apparently similar mesenchymal cell could on 

 account of its internal difference give rise to erythroblasts. 

 It is a little strange at least that the white blood cells arise far 

 interstitially, while the erythroblasts have such a decided tend- 

 ency to proliferate into the sinusoids or vascular spaces if they 

 both arise from a common stem cell. The two environments in 

 which they develop could scarcely account for the differences 

 between red and white corpuscles, since in the body of an em- 

 bryo in which the blood circulates there are several places where 

 the two types of cells develop side by side, as Maximow and 

 others have described. The reasons for the differences are the 

 internal differences between the mesenchymal cells from which 

 the two types of corpuscles arise. 



The white blood cells and red blood cells, although both are 

 derived from mesenchyme, arise from mesenchymal cells which 

 have already differentiated sufficienctly far not to be inter- 

 changeable. This statement is probably true also of the vascu- 

 lar endothelium in its relationship to blood forming mesenchyme. 

 The embryonic mesenchymal cell if taken in an early enough stage t 

 could no doubt give rise to other mesenchymal cells which 

 would later form any of these different type cells. When, 

 however, differentiation has proceeded to some degree, sufficiently 

 far to form what is termed by embryologists an organ anlage, 

 and yet not far enough to make it possible to distinguish between 

 the appearance of various mesenchymal cells, they are then, 

 nevertheless, different in their potentialities. 



