THE HISTOGENESIS OF DENSE LYMPHATIC TISSUE 193 



Presupposing the stem cells of the erythroblastic line of de- 

 velopment to be elements of a lymphocytic nature (lymphoid 

 hemoblasts) which are morphologically the same as the stem 

 cells of the granuloblastic line of development, the question is 

 raised as to what the factors are causing morphological similar 

 cells to difTerentiate in such entirely different ways. Do the stem 

 cells, as some polyphyletists say, possess only morphological 

 similarity, with some inherent differences in them, by reason of 

 which each must develop along a certain line? It seems more 

 feasible to attempt to explain this difference in development by 

 changes in environmental conditions, which would affect the 

 metabolic activity of the cell. 



In forms below the mammals a decided difference in environ- 

 mental conditions is noted . Maximow has shown that erythrocytes 

 develop intravascularly and granulocytes extravascularly in 

 amphibia and selachians. It has been observed in studies 

 of blood-cell development in birds and reptiles that almost 

 invariably the erythropoietic foci are located intravascularly, the 

 granulopoietic foci extravascularly (van der Stricht, Bizzozero, 

 Danchakoff, and others). This relationship has been most care- 

 fully studied by Danchakoff. She did not, however, as van der 

 Stricht, think the presence of an endothelial wall as sufficient 

 evidence of the separate origin of these two types of cell. 



If this were invariably true that erythropoiesis occurs intra- 

 vascularly and granulopoiesis occurs extravascularly, that differ- 

 ence in environmental conditions would surely be sufficient to 

 account for the different development of these cells; but, un- 

 fortunately, there seems to be an exception to this rule in the 

 case of mammals, for it is a commonly accepted fact now among 

 hematologists (except van der Stricht and a few others) that 

 erythropoiesis occurs in large part extravascularly in mammals. 



But if one finds similar cells under exactly the same environ- 

 mental conditions developing into totally different types of cells, 

 it strengthens the belief that there must be some inherent differ- 

 ences between these apparently similar stem cells. Stockard 

 ('15) arrived at this conclusion after noting the extravascular 

 formation of both types of blood cells in mammals. He found 



