EQUIVALENCE OF HEMATOPOIETIC ANLAGES. I. SPLEEN 289 



Development of intense erythropoiesis after stimulation. — The 

 stimulation of the lymphoid hemocytoblasts to intense prolifera- 

 tion may influence the development of the vessels and of their 

 contents in the mesenchymal spleen anlage. As above stated, 

 the stimulation in early stages leads often to defective develop- 

 ment of veinous sinuses and large parts of the spleen anlage are 

 transformed into granuloblastic tissue. If stimulation is ap- 

 plied a little later, at the ninth day, when a number of sinuses 

 are already formed, others still appear, a development of intense 

 erythropoiesis may be observed in the spleen anlage. The nor- 

 mal spleen tissue at this stage consists of a mesenchymal tissue, 

 in which numerous hemocytoblasts continue to develop. The 

 stimulation of the lymphoid hemocytoblasts and the simul- 

 taneous opening of splits in the mesenchymal tissue leads to the 

 appearance of numerous hemocytoblasts within the vessels. 

 Larger groups of lymphoid hemocytoblasts may occupy the 

 lumina of the sinuses and here undergo an erythroblastic dif- 

 ferentiation. Figure 15 shows a similar split developed locally 

 in the spleen mesenchyme. It is of irregular form and sur- 

 rounded by mesenchymal cells. These cells enter as a constit- 

 uent part into the general spleen tissue and send some of their 

 processes within the lumen of the sinus. A development of 

 numerous lymphoid hemocytoblasts is observed in this tissue 

 (fig. 15 E.L.Hhl.) and large groups of l^^mphoid hemocytoblasts 

 are discharged in the lumen of the sinuses (fig. 15, I.L.HbL). 

 Outside the vessels the lymphoid hemocytoblasts differentiate 

 into granulocytoblasts (fig. 15, Grhl.), as elsewhere, within the 

 ^'essels they develop into erythroblasts (fig. 20, Erbl.) and 

 erythrocytes {Ere). The process of the normal differentiation 

 of a lymphoid hemocytoblast into an erythrocyte was studied 

 in birds by Danchakoff (9) and corresponds to what is seen 

 under the condition of an experimental stimulation. Therefore 

 I refer in this respect to my previous papers on the develop- 

 ment of different hematopoietic organs in birds and reptiles. 



Figure 16 shows the spleen tissue in later stages (four days 

 after stimulation of an eight day embryo) . The veinous sinuses 

 appear surrounded by flattened mesenchymal cells. The inner 



