20 MYELOID METAPLASIA OF THE EMBRYONIC MESENCHYME. 



attain. Five days have passed in this case after the grafting of adult splenic tissue 

 on the allantois of this embryo. The proliferation and transformation of numerous 

 mesenchymal cells into mobile elements, which may be readily observed in the first 

 2 or 3 days after grafting, have led to remarkable changes in the stroma of the 

 mesonephros. In figure 14 the glomerule and the various tubules are widely sep- 

 arated by a dense tissue, which is traversed by numerous capillaries. This tissue 

 consists, for the greater part, of amoeboid granular cells easily identified with the 

 various stages of granuloblast^ differentiation of the hemoblast. Most of them are 

 granuloblasts (myelocytes), though both completely differentiated granular leuco- 

 cytes and numerous younger stages of hemoblasts are also found in these huge 

 accumulations. Typical mesenchymal cells are scarce and in some regions would 

 seem to hardly suffice to preserve the coherence of the organ. It is remarkable 

 how an accumulation of renal tubules separated by spaces filled with amoeboid 

 cells can still hold together. Though the development of the granuloblasts tissue 

 may lead to huge granuloblast^ accumulations, no formation of necrotic foci was 

 observed, as described in the allantois, the muscles, and the spleen. 



Not all of the organ may show such intense changes as illustrated in figure 14. 

 In the central parts of the organ, where the mesenchyme originally is scarce, its 

 proliferation and further differentiation never leads to the development of such 

 heavy strands of granuloblast^ tissue as are seen in this figure. Here the different 

 segments of the renal tubules are much closer together and the lack of space may of 

 itself prevent a more intensive proliferation of the mesenchyme. Some of the 

 tubules are separated from each other by just a few fibrils; in this case, of course, 

 they will retain their respective positions. But wherever mesenchyme is present, 

 no matter how small the interstices between the windings of the renal tubules may 

 be, a proliferation and further differentiation of the mesenchyme situated in these 

 interstices regularly leads to the formation of larger and smaller islands and strands 

 of granuloblast^ tissue. The granular leucocytes and the granuloblasts are very 

 mobile cells, and it is not a rare occurrence to find some of them traversing the walls 

 of the vessels or situated within the lumen of the renal tubules; a couple of granular 

 leucocytes may be seen in the lumen of a tubule in figure 14. 



The study of the changes developing in the mesenchyme of the mesonephros, 

 after grafting adult splenic tissue on the allantois of the embryo, has revealed 

 hemopoietic potentialities in a new region of embryonic mesenchyme. The mesen- 

 chymal cells in the mesonephros during the functional period of this organ have 

 lived in intimate symbiotic relations with the variously organized renal epithelia. 

 The stimulation brought about by the presence of the splenic graft coincides with 

 the beginning of the involution of this organ. The specific activity exercised by 

 the renal epithelium might have exhausted the synthetic power of the cells, or else 

 products elaborated during development of various organs in the embryo have in- 

 jured it to such a degree that it degenerates and undergoes a final disintegration 

 and resorption; or, finally, special vascular changes in the region of the meso- 

 nephros might have had an unfavorable effect on the organ. No definite data are 



