MYELOID METAPLASIA OF THE EMBRYONIC MESENCHYME. 7 



mesenchymal constituents in adult organs, under pathological conditions, into 

 various kinds of blood-forming tissue have raised the question as to whether these 

 results were due to a diverse activity of mesenchymal cells, polyvalent even in the 

 adult, or to the proliferation of specific cell-groups present in the organs or brought 

 in as cellular emboli. 



On the basis of the descriptive investigations, the potentialities of the loose 

 mesenchyme can not be definitely established. Its partial development into blood- 

 cells, together with the lack of a strongly defined localization of the hemopoiesis 

 in the ontogenetic development, as well as in its phylogenetic evolution, have served 

 alternatively as arguments for its polyvalency or for its specificity. The invari- 

 able differentiation of a part only of the mesenchyme into blood-cells can indeed be 

 interpreted either as a result of the specification of definite mesenchymal cell-groups 

 or as a manifestation of a general potency, dependent upon factors under whose 

 control a part only of the mesenchyme is placed. Only by experimentally extend- 

 ing the blood-forming activity to parts of the mesenchyme, which, under normal 

 conditions, never manifest similar potencies, can we prove that they possess them. 

 Only by experimentally calling forth in mesenchymal cells a potency different from 

 that revealed by them under normal conditions can we prove that they are poly- 

 valent. Departure from the typical development manifested under experimental 

 conditions by either mesenchyme or first blastomeres can be compared to the dif- 

 ferences observed in the fall of a feather in vacuo and in air. 



In the two parts of the "Equivalence of Hematopoietic Anlages" (1916e, 19186) 

 it was my purpose to analyze the changes called forth by grafting of adult splenic 

 tissue on the chick allantois in the spleen of the host, a merely specialized con- 

 densation of the mesenchyme, and to compare them with those which necessarily 

 must have taken place in the loose mesenchyme of the allantois itself, if the mesen- 

 chyme in the allantois were equivalent to that of the spleen. In order to establish 

 whether or not the mesenchymal part of the spleen has changed its potentialities 

 with age, an analysis was also made of the fate of the adult splenic tissue grafted on 

 the chick allantois. In a previous paper (1916cT) changes in the structural char- 

 acters of the thymus consequent upon the same experimental intervention were 

 recorded. The results of this study clearly indicate that mesenchymal cell-groups 

 in such different embryonic regions as thymus, spleen, and allantois can be simul- 

 taneously brought to a proliferation, of which the intensity strikingly surpasses 

 the proliferative capacity of the usual embryonic mesenchymal tissue. This simul- 

 taneous proliferation was incited by the same factors brought about by the grafting 

 and growth of adult splenic tissue on the allantois of the embryo. Moreover, the 

 proliferation of the mesenchyme in all these regions was followed by an analogous 

 differentiation. Mesenchymal cells lost their syncytial arrangement and appeared 

 in groups of amoeboid cells, which in the literature were given various names— 

 stem-cells, hemoblasts, hemogonia, mesamceboid cells, etc. These cells continued 

 to proliferate and, according to their topographic localization, finally were trans- 

 formed into granuloblasts or, if situated in the venous sinuses of the developing 

 spleen, into erythroblasts. 



