196 JOHN STEPHENS LATTA 



of the current, together with the thinness of the vascular walls, 

 usually only a lining endothelium, affords excellent opportunity 

 for the transudation of substance^ from the blood stream to the 

 outlying near-by lymphoid hemoblasts. 



It seems very probable, though, that the complete explana- 

 tions for these erythropoietic tendencies are deeply seated. 



Elements are present in the blood stream in certain definite 

 proportions. A disturbance of this balance between elements 

 in any way, as by change of conditions, destruction of some of the 

 elements, etc., causes initiation of erythropoiesis to restore that 

 balance. The submucosal connective tissue in these regions 

 affords an excellent place for this to occur, for here stem cells 

 (lymphoid hemoblasts) are being produced, the current of the 

 blood stream is slow, the blood vessels are numerous, and the 

 vascular walls, to a great extent, quite thin. Then, too, the 

 connective tissue is here performing no very active function, and 

 ample space is provided between the bases of the nodules and the 

 muscle coats (especially in Peyer's patch) for the development 

 of erythropoietic foci. 



DISCUSSION OF THE RELATIONS CONCERNED WITH THE FORMA- 

 TION OF THE BLOOD CELLULAR ELEMENTS 



The formation and development of the three different types of 

 blood cells, then, and relations causing or connected with the 

 appearance of each type evidently are very closely associated. 



Lymphopoiesis, granulopoiesis, and erythropoiesis are all 

 closely associated with the vascular system. Lymphopoiesis, of 

 course, must be the first process to occur, for it is from stem 

 cells of a lymphoid nature that the cells of the granuloblastic 

 and erythroblastic series are developed. This lymphopoietic 

 process is, apparently, in some way, initiated by the lymphatic 

 vessels, free lymphoid cells (lymphoid hemoblasts and small 

 lymphocytes) forming by differentiation of mesenchymal cells. 

 The subsequent fate of these cells, thus formed, is dependent 

 upon the blood supply and the closeness of their relation with the 

 blood vessels. If there seems to be no particular association 

 with the blood vascular system, these free cells continue to 



