EQUIVALENCE OF HEMATOPOIETIC ANLAGES. I. SPLEEN 283 



under normal conditions should be regarded as necessarily 

 determined. The organ develops in a complex environment of 

 other organs which proliferate and differentiate at the same time. 

 Each of these factors, which necessarily ensues from the collective 

 action of all the surrounding conditions, becomes itself one of 

 the factors which partly impels the results of the general differen- 

 tiation. The necessity and mutual dependence of the develop- 

 ment processes result from reciprocal influence of the factors and 

 is characterized by striking purposefulness. The general out- 

 lines of the development may therefore be changed, as will be 

 shown later, only in the case if the regular interaction of factors 

 has been disturbed. 



Variations in the intensity of differentiation of various cell 

 groups may be observed, however, in normal spleens. Numerous 

 or merely scant lympho- and granulocytoblasts may develop at 

 the expense of mesenchymal cells. The proUferation of granu- 

 locytoblasts may also be more or less intensive. But this differ- 

 entiation process is extended in the spleen anlage more or less 

 uniformly at a time when the anlage presents a homogeneous 

 structure and is lacking special conditions of vascularization. 

 The differences in the extent of the normal granulopoiesis are, 

 however, not excessive and deviation in other lines of differen- 

 tiation may compensate them. A new line of differentiation 

 necessarily starts with the appearance of new structural 

 characters, namely with the development of the arteries. After 

 this new factor has been ' established, it will permanently 

 influence the cells, which stand under its control. The existence 

 of regular unchangeable relations between definite structural 

 conditions and differentiation of certain kind of cells seems to 

 explain sufficiently the necessity which appears in a group of 

 identical stem cells to diverge in their further development. Is 

 there a need of recurring to invisible differences between cells, 

 lest it should be necessarily required by deduction from the 

 results of development? 



One phase in the spleen development could, however, be inter- 

 preted by the dualists in their favor— this is the apparently late 

 ingrowth of the arteries in the pulpa-like spleen anlage. If both 



