EQUIVALENCE OF HEMATOPOIETIC ANLAGES. I. SPLEEN 281 



the 15 to 17 days of incubation. They increase their number by 

 differentiation at the expense of lymphoid hemocytoblasts and 

 by their own proliferation. They gradually infiltrate the regions 

 with arterial vascularization; finally they accumulate in dense 

 masses around the arteries. One may observe amongst them a 

 few degenerated cells, even in an embryonic spleen; they are 

 phagocytosed by mesenchymal and endothelial cells and some- 

 times even by hemocytoblasts. A number of mesenchymal and 

 endothelial cells are gradually transformed into typical macro- 

 phages (Evans) (15). Similar macrophages are observed also 

 in the lumen of the sinuses, where they lie free, or form a part 

 of the surrounding mesenchymal tissue (fig. 17, 18, 19), In the 

 pulpa, however, the activity of the macrophages is directed chiefly 

 toward the degenerated erythrocytes. Though at this time the 

 connection between the arterial and the veinous vessels is com- 

 pleted, these regions continue to be very distinct. 



The appearance of the follicles and of the new line of cell differ- 

 entiation does not seem to influence the life of the cells in the 

 pulpa. Both the granuloblastic and the lymphoblastic processes 

 of differentiation coexist now in the spleen, and are displayed by 

 lymphoid hemocytoblasts in different regions under different 

 structural conditions. The granulo- or leukopoiesis develops 

 around the large veinous sinuses under conditions identical to 

 those under which they develop in the yolk sac and in the bone- 

 marrow. The leukopoiesis in the spleen is much reduced after 

 the embryo is hatched, and the spleen becomes chiefly a lympho- 

 poietic and erythrolytic organ. 



The structural peculiarities which determined the various lines 

 of differentiation of the polyvalent stem cells remain in the 

 adult spleen unchanged. Different stimulating agents may cause 

 a proliferation of the stem cells. Their differentiation, however, 

 will correspond to the structural environmental conditions to 

 which they are submitted. It is therefore only natural that 

 the myeloid metaplasis in the adult spleen should develop 

 in the pulpa, where the structural conditions determine 

 normally a granuloblastic differentiation of the hemocytoblast. 

 A differentiation of small lymphocytes in the pulpa, or vice 



