290 VERA DANCHAKOFF 



surface of the sinuses appears even and regular and is covered 

 by cells which resemble endothelial cells. The vessels still con- 

 tain a large number of young undifferentiated blood cells which 

 continue to proliferate and to differentiate. The tissue between 

 the vessels begins to assume the character of a reticular tissue, in 

 the meshes of which numerous free ameboid cells are lying. 

 They chieflj^ consist of lymphoid hemocytoblasts, granulocyto- 

 blasts, and granulocytes (fig. 16, E.L.Hbl., 6rhl, Grc) together 

 with cells at intermediate stages of differentiation. However a 

 few macrophages may occasionally be seen. 



The intense development of erythropoiesis and the activation 

 of granulopoiesis convert the spleen at this stage into a true 

 myeloid organ. As above stated, an occasional differentiation of 

 lymphoid hemocytoblasts into erythroblasts may be observed 

 during normal development of the spleen. Under stimulation 

 large groups of lymphoid hemocytoblasts fall into the developing 

 sinuses. At this time the vascularization in the spleen appear as 

 a net of large veinous capillaries annexed to the portal system and 

 the blood current must be here very slow. Situated in the sinus 

 lumen the lymphoid hemocytoblast is not conveyed away by 

 the blood current, but it remains in the sinus, proliferates and 

 differentiates into a red blood cell. The conditions of the de- 

 velopment of the erythropoiesis in the spleen correspond to those 

 known in the 3'"olk sac annexes and in the bone-marrow. A de- 

 velopment of large veinous capillaries with a slow blood current 

 and a close connection between erythro- and granulopoiesis 

 is seen in all these regions. 



However, the erythropoiesis incited by the experimental stimu- 

 lation does not persist in the spleen permanently. When the 

 arterious vascularization develops and the connection with the 

 sinuses is effected, the blood current becomes swifter, the younger 

 cells are withdrawn and gradually replaced by differentiated 

 erythrocytes. In other cases in which the arterial vasculari- 

 zation develops defectively and no regular food supply is estab- 

 lished a lack of nutritive material finally may determine the 

 suspense of erythropoiesis. Large macrophages develop in the 

 sinuses and an intense phagocytosis of erythrocytes may be 

 observed (figs. 17, 18, 19). 



