ORIGIN OF BLOOD AND ENDOTHELIUM 229 



they have become so mixed and confused that it is almost im- 

 possible to separate the cell groups or differentiate between 

 them. One must in this connection, remember the fact that 

 almost all authors have concluded that only red blood cells are 

 formed in the blood islands of the yolk-sac in most vertebrates. 

 All authors who have studied the development of the blood 

 in Teleosts have invariably described only red blood cells as 

 arising in the intermediate cell mass. No one has ever men- 

 tioned the presence of white blood cells in the true blood forming 

 anlage of the Teleosts. 



The monophyletic school really goes still further and not only 

 claims that all types of blood cells arise from a common mother 

 mesenchymal cell, but also that the vascular endothelial cell is 

 likewise capable of giving rise to the various types of blood 

 cells and is originally a cell of the same type as the stem mother 

 cell. There are numerous descriptions and illustrations of the 

 origin of blood cells from the vessel linings in the literature of 

 the past twenty-five years, since Schmidt in 1892 described 

 the transformation of individual endothelial cells into white 

 and red blood corpuscles. Yet again, I believe that the really 

 skeptical reader will not be at all convinced that such a thing 

 ever takes place from the evidence presented in the literature, 

 certainly not from any of the illustrations that have been made 

 of this process. 



No real vascular endothehal cell has been actually observed 

 to metamorphose into a blood cell or to divide off another cell 

 which forms a blood cell, and until such a direct observation is 

 forthcoming one can only question the accuracy of the inter- 

 pretation of the various observations up to now recorded. 



The mesenchyme is a very generalized embryonic tissue and 

 from it arise the various kinds of blood cells, endothehal cells, 

 connective tissue cells, etc. There can be no doubt of the 

 great genetic difference between blood cells and connective tis- 

 sue cells, yet their parent cells are with our present methods 

 indistinguishable. We may with equal justification go further 

 and hold Ukewise that the cells from which the vascular endothe- 

 lium, red blood cells and white blood cells arise are mesenchymal 



