EQUIVALENCE OF HEMATOPOIETIC ANLAGES. I. SPLEEN 297 



tlie development of mesenchyme and lymjilioid liemocytoblasts 

 can be intensely stimulated and that the respective boundaries, 

 in which the differentiation of certain cell groups usually takes 

 place, can be shifted (displaced), in other words, that the pro- 

 spective potency (after Driesch's terminology) of the blood stem- 

 cells is greater than their prospective value. 



The fact that under certain conditions nearly the whole amount 

 of the mesenchymal cells of the spleen anlage may undergo a 

 granuloblastic differentiation; that under other conditions they 

 show a fibroblastic, or an erythroblastic differentiation, and so 

 forth, is the natural sequence of the polyvalency of the m.esen- 

 chymal cells in the spleen anlage. The early process of segre- 

 gation must have led in the spleen anlage to a production of one 

 group of numerous homogeneous stemcells, which under different 

 conditions split off varioush' differentiated cells. 



Data concerning the conception of cell differentiation 



The histogenetical studies of the spleen under normal con- 

 ditions and after stimulation have shown that a mesenchymal 

 cell, which normally would contribute to the formation of retic- 

 ulum tissue, can develop into a hemocytoblast or into a fibro- 

 blast or a giant cell, that the lymphoid hemocytoblasts can 

 develop into a granular leucocyte, or into an erythrocyte or a 

 small lymphocyte. The study of hematopoiesis in birds and 

 reptiles shows very definite conditions for each of the lines of 

 differentiation. Though Haff (18) has lately described the 

 presence of extravascular erythropoiesis in the liver of hens, 

 yet I was not able to confirm his data. My personal obser- 

 vations concerning the existence of small centers of erythropoiesis 

 in the connective tissue of the hen do not seem to contradict 

 the general conditions. These centers, scattered irregularly 

 and finally phagocytosed may be interpreted as locally developed 

 blood and vessel anlages, of which the connection with the general 

 circulation has not been fully effectuated. 



It is known, however, that both erythropoiesis and granulopoi- 

 esis develop in mammals extravascularly and, according to 

 Maximow (25) ('11) — "zweineben einander liegende ganz gleiche 



