22 MYELOID METAPLASIA OF THE EMBRYONIC MESENCHYME. 



definitely. It is true that the factors for the granuloblast ic differentiation of the 

 mesenchymal stroma become, under the present experimental conditions, so powerful 

 as to transform a large amount of mesenchymal cells into granuloblasts even in the 

 regions of the organism in which granulopoiesis has never occurred in a typical 

 development. It would therefore be only natural to expect that, if the nephrogenic 

 tissue proper possessed a potentiality for granuloblastic differentiation, it would 

 have manifested it under the present conditii >n. The granuloblastic potentiality may, 

 however, still be a property of the nephrogenic tissue, but the factors for the specific 

 organization of the nephrogenic tissue into renal epithelial tubules which arc 

 unknown to us may be of a more decisive nature for their realization. 



The morphologically homogeneous mass of mesenchyme of which the nephro- 

 genic tissue consists, and which is organizing itself on two different lines, may on the 

 other hand prove to be of a different nature. A special study of its minute structure 

 by various methods may possibly discover morphological differences which will 

 account for the various lines of differentiation. The present study shows only 

 that a part of the metanephric primordium (which under normal conditions remains 

 inert and in the final stages of the development of the metanephros is represented 

 by scanty connective-tissue cells), at the stage of 7 to 9 days of incubation, maybe 

 stimulated to hemopoietic activity and that its total potencies therefore are greater 

 than those it is revealing under normal embryonic conditions. 



It is a different question as to how long these mesenchymal cells are retaining 

 the latent potencies. Maximov, in his paper on development of bone and bone-marrow 

 in the kidney of the rabbit (1907) does not admit that the scanty connective-tissre 

 cells of the stroma in the kidney possess hemopoietic potentialities. The hcne- 

 marrow, developing in the kidney after ligature of the renal vessels, is for him not of 

 local origin, because the "sehr zellarmessparliehesinterstitielles Bindegewebeentahlt 

 im normalen Zustande sicher keine Spur von myeloiden Elementen. Dass aus 

 gewohnlichen Bindegewebszellen, Fibroblasten, Myeloidgewebe enstehen konnte 

 scheint Niemand ernstlich zu glauben." 



I endeavored to solve this problem by transplanting adult kidney tissue on the 

 allantois of the embryo. Here the mesenchymal constituents of the adult spleen 

 have been seen to transform into hemopoietic, or, more specifically, into granu- 

 lopoietic tissue. It was hoped that the behavior of the stroma of the adult kidney 

 in the allantois might give some further information about its potentialities. The 

 experiments carried out gave very decisive data concerning the ability of the renal 

 epithelium to further proliferate, its tenacity to retain its epithelial arrangement, 

 certain phases of its function, and also as to the great digestive power of the 

 mesenchymal and endothelial cells situated between the renal tubules. No defi- 

 nite data were obtained concerning the differentiative ability of these cells. A 

 detailed account of the growth of adult and embryonic kidney tissue on the allan- 

 tois can not be presented here; a few words will suffice for our present purpose. 



The growth of the transplanted adult kidney tissue has never been observed 

 to be as intense as that of the embryonic kidney. It was observed, moreover, 



