MYELOID METAPLASIA OF THE EMBRYONIC MESENCHYME. 23 



that after 10 to 12 days of growth on the allantois the grafts never contained 

 any necrotic tissue; but a study of the earliest stages (1 to 4 days of growth) revealed 

 that only those renal tubules and glomerules survived which were adjacent to the 

 embryonic mesenchyme of the allantois. A great number of them not so favorably 

 situated appeared necrotic after 2 days of their existence in the allantois. The 

 epithelial cells of all these tubules died and disintegrated. The blood-cells present 

 at the time of the graft in the capillaries also succumbed. The fate of the vascular 

 endothelium and the scanty connective-tissue cells in the interstices between the 

 renal tubules and the lumen of the vessels was different. They now appeared 

 swollen, their bodies well limited, sometimes widely separated from each other, their 

 nuclei enlarged and oval, their cytoplasm intensely basophilic. At present I can 

 not go into the details of the process, but there is no doubt that an active diges- 

 tion of the necrotic epithelial cells and of the blood-cells took place by the surviv- 

 ing cells, the vascular endothelium, and the connective-tissue cells. I do not have 

 at present any criteria which would enable me to distinguish them in relation 

 to their digestive power; 2 and 3 days after the grafting both kinds of cells 

 appeared intermingled and their specific morphological characters were no longer 

 distinguishable. Though the renal epithelium seemed to undergo extracellular 

 digestion, and blood-cells were in great numbers incorporated in the cytoplasm 

 of the cells, no distinction could be made between the endothelial cells and the 

 connective-tissue cells in relation either to structure or digestive capacity. 



The digestive power of the endothelial and connective-tissue cells lying between 

 the renal epithelial tubules completed the elimination of the dead cells, blocks of 

 unaltered protein, 3 to 5 days after grafting; that is why I never encountered any 

 necrotic foci in later stages of kidney grafts; 3 to 5 days after grafting, however, 

 numerous embryonic mesenchymal cells grew from the allantois into the graft and 

 it was no longer possible to distinguish between the embryonic and the adult cells. 

 Occasionally single cells, in other places large groups of them, detached themselves 

 from the common syncytium and underwent a granuloblastic differentiation, but 

 it could no longer be decided whether these foci of granuloblastic tissue had their 

 origin in the connective tissue of the transplanted adult kidney or whether they were 

 derived from the ingrown mesenchyme of the embryo. Therefore I must leave it 

 undecided whether or not the endothelial and connective-tissue cells of the adult 

 kidney possess a hemopoietic potentiality. The study of their fate in grafts of 

 adult kidney has, however, demonstrated their great digestive power. 



MESENCHYME IN OTHER REGIONS OF THE EMBRYO BODY. 



There is no need of a detailed analysis of changes observed after grafts in the 

 mesenchyme in other regions of a 7 to 8 days' embryo. A brief survey will be 

 sufficient in order to show that they are analogous to those already described in the 

 muscles, sex-glands, kidneys, etc., and vary only with the relative scarcity or 

 abundance of mesenchyme in a definite region of the embryo body at that time. 



Liver, pancreas, suprarenal glands, intestine, the axis of the developing 

 feathers, the connective tissue of the derm, the adventitia of the vessels, and the 



