EQUIVALENCE OF HEMATOPOIETIC ANLAGES. I. SPLEEN 263 



It was repeatedly pointed out that the stem cells, which in the 

 })ulpa and in the follicle of the spleen give different products of 

 development, seem to bear a perfectly similar morphological 

 structure, Dominici (10), Hirschfeld (21), Meyer u. Heinecke 

 (26), Weidenreich u. Downey (12), Butterfield (5), and others. 

 However, many recent data seem to indicate that the isomorph- 

 ism does not imply either isogenesis nor especially isodynamics. 



The study of myeloid metaplasis in such organs, in which both 

 directions of differentiation coexist permanently in the adult 

 organism or temporarily in the embryo, does not evidently offer 

 favorable opportunity of solving the problem : whether the adult 

 organism does preserve a stock of embryonic undifferentiated 

 cells capable of various differentiation, or not. Neither do 

 these studies solve the question: whether the morphological 

 structure so characteristic of and common to the young cells 

 both of the lymphatic and myeloid tissue is a result of definite 

 physico-chemical constitution of which further changes are im- 

 plied by differences in environmental conditions; or whether a 

 group of morphologically identical cell units may have different 

 physico-chemical constitution which would imply their further 

 different development. 



My (9) personal studies on the normal histogenesis of the 

 ■ blood-cells and of the hematopoietic organs in birds and reptiles 

 led me to a monogenetic conception of their origin. The study 

 of regeneration of hematopoietic tissue after bleedings as well 

 as that of changes undergone by this tissue during starvation 

 (9) seemed to corroborate the monogenetic interpretation. 



The admission of the existence of common stem cells in differ- 

 ent hematopoietic organs implies therewith the admission of 

 identical reaction of these stem cells to a stimulating agent as 

 far as these stem cells are submitted to the same conditions. 

 The structural environment in the full-grown organism is, 

 however, highly differentiated in the various organs. The 

 stem cells in different hematopoietic organs are of course expected 

 to respond to stimulation by simultaneous proliferation; but their 

 differentiation will be specific according to environmental condi- 

 tions, for their differentiation depends upon conditions, usually 



