284 VERA DANCHAKOFF 



arteries as well as veins developed in loco, there would remain 

 no doubt in the common origin of the lymphatic and the myeloid 

 tissues. The ingrowth of the arteries may be regarded as a mere 

 stimulus for a new intense proliferation of the mesenchyme and 

 its subsequent differentiation into small lymphocytes. On the 

 other hand, the ingrowing vessels might have brought new cell 

 material for a new Une of differentiation, which starts at this 

 period. It is difficult to establish with certainty whether the 

 mesenchymal cells of the follicle anlages develop at the expense 

 of the local mesenchyme, which remains undifferentiated, or 

 whether they are derived from the tissue brought by the 

 growing arteries. No matter how this question will be solved, 

 both in the pulpa and in the follicles, the mesenchymal cells 

 bear the same morphological structure. They present moreover 

 in their first differentiation stages undeniable analogies. In 

 the pulpa as well as in the folhcles they partly differentiate into 

 lymphoid hemocytoblasts and partly form the so-called reticular 

 tissue. The special differentiation according to environmental 

 conditions is then undergone by the lymphoid hemocytoblasts. 



The dualists admit that the development of the myeloid 

 metaplasis of the spleen, which under definite pathological 

 conditions is localized in the pulpa is due to proliferation and 

 differentiation of adventitial or endothelial vascular cells. Endo- 

 thelial cells are present both in the pulpa and in the follicles, 

 the adventitial cells are even more numerous in the arteries of 

 the follicles. If the strain in the fm-ther differentiation of the 

 lymphoid hemocytoblast has to be laid in the cell itself and not 

 in the physico-chemical conditions, given by structural peculiar- 

 ities of the environment, the myeloid metaplasis should have ap- 

 peared as a diffuse process, for endothelial and adventitial cells 

 are found in the pulpa as well as in the follicles. In order to 

 remain consequential, the dualists ought to recognize various 

 kinds of adventitial and endothelial cells — the ones being capa- 

 ble of a myeloid differentiation, the others not. 



The striking dependence of the various differentiation of 

 morphologically identical cells upon a change in the environ- 

 mental conditions may greatly strengthen the monogenetic 



