8 MYELOID METAPLASIA OF THE EMBRYONIC MESENCHYME. 



The mesenchyme of the thymus, of the spleen, and of the allantois was shown 

 to possess similar potentialities which, in a typical development, were either realized 

 in a slighter degree or not revealed at all. Large areas of mesenchyme were in all 

 these organs transformed into granuloblastic tissue. Microscopical characteristics 

 of the organs (for example, the size of the spleen and thymus) also were much 

 modified, the organs under experimental conditions increasing in size to many 

 times the normal average. Finally, the analysis of the fate of the grafted adult 

 spleen brought quite definite evidence for the retention by the adult splenic mesen- 

 chyme of its original potencies — the cellular reticulum of both follicles and red pulp 

 underwent granuloblastic transformation. Not only, therefore, could the size, and 

 especially the structural character, of such organs as thymus and spleen be changed 

 during their development in the embryo, but the structure of a normal adult spleen, 

 of which the hereditary characters seemed to have been fully realized, could be 

 radically changed by transplanting parts of this organ into the allantois. 



The microscopical study of these transformations has shown that these changes 

 depended upon a modified activity of polyvalent mesenchymal cells, which 

 responded in definite but different ways to normal and pathological stimuli. The 

 latter, introducing into the environmental conditions factors which were not present 

 in normal typical development, must have brought changes in the metabolism of 

 the mesenchyme which now liberated numerous free cells, hemoblasts. These 

 factors called forth an intense differentiation of hemoblasts into granular leucocytes 

 in regions in which it normally occurred only in a moderate degree, and also 

 incited it anew in areas in which normally no similar differentiation was found. 

 In the light of these results the embryonal mesenchyme of the spleen, of the 

 thymus, and of the allantois of the chick has to be regarded as possessing equal 

 power for granuloblastic differentiation. 



During the early stages of splenic development the stem-cells showed them- 

 selves capable of various differentiation, while their fate was determined by whether 

 they were incorporated into a developing venous sinus or were left outside. The 

 stem cells in the thymus, which under normal conditions develop into small 

 lymphocytes, were seen to follow a granuloblastic line of differentiation. More- 

 over, the mesenchyme of the adult spleen in the form of the cellular reticulum — a 

 source for the small lymphocytes in later embryonic and adult stages — was shown 

 to retain its potencies for granuloblastic differentiation, thus clearly demonstrat- 

 ing the polyvalency of the mesenchyme in these regions. 



Definite as these findings may appear as experimental proofs for the polyvalency 

 of the loose mesenchyme, they still leave a large, unsettled field in relation to 

 numerous other regions in which the mesenchyme is found in the organism. Are 

 not the above cited regions of mesenchyme centers of predilection in which granulo- 

 poiesis may be developed in embryonic stages and rekindled, even in adult? The 

 splenic mesenchyme develops normally a granulo-poietic activity, which in intensity 

 may vary considerably in different specimens. Granulopoiesis in the thymus exists 

 normally in reptiles and is occasionally observed in other animals. No granulo- 



