MYELOID METAPLASIA OF THE EMBRYONIC MESENCHYME. 19 



supported by facts. Nevertheless, such a conclusion can be justly applied only to 

 the results of typical development; as shown by the present observation, it is only 

 in a typical development that the mesenchyme of the kidneys in higher vertebrates 

 fails to manifest hemopoietic potencies. On the basis of the typical development of 

 the mesenchyme no decision can be made whether it has undergone a definitive 

 specification or whether on account of unfavorable environmental conditions, it is 

 not revealing its hemopoietic potencies, though it possesses them in a latent state. 



The study of the mesenchyme in the mesonephros and metanephros in chick 

 embryos with a generalized granuloblastic differentiation of the mesenchyme 

 clearly shows that in both organs it may enormously proliferate and undergo a 

 further differentiation into granuloblastic tissue (at least at the stage of 7 to 9 days) . 

 The proliferative processes observed in the mesenchyme of the mesonephros and 

 metanephros respectively, though essentially similar, bear well-pronounced individ- 

 ual characters depending upon the different environment in which they develop and 

 demand a separate description in each case. Both organs are offspring from the 

 same source. The lines of their development or differentiation are closely similar 

 and yield distinctly analogous results. The differences in the immediate environ- 

 ment, which influence the proliferative processes of the mesenchyme in both organs, 

 depend, therefore, not upon the different nature of tissues with which the mesen- 

 chyme enters into a correlation, but upon different developmental stages of the renal 

 epithelial tissue. The stimulation of the mesenchyme and its first response in the 

 embryos under experiment took place at 7 to 10 days of incubation. At this period 

 the mesonephros has not only fully developed, but has already begun its involution, 

 while the proliferative and different iative processes in the metanephros are pres- 

 ently at their height. This difference may well explain the quantitative varia- 

 tions in changes observed in the mesenchyme of the mesonephros and metanephros 

 after grafts. 



Mesenchyme of the mesonephros. — The changes occurring in the mesenchyme 

 of its caudal part only (the derivative of the nephrogenic cord) were studied. The 

 first changes observed in the mesonephros after grafts do not require much comment, 

 for they are identical with the proliferation and separation into mobile cells under- 

 gone by the mesenchyme in other regions of the embryonic body and already 

 described for the mesenchyme of the allantois and of the muscles. Certain pecu- 

 liarities are imposed on the process by the scarcity of space assigned at this time 

 in the mesonephros to the mesenchyme, which, together with the capillaries, occupy 

 the very narrow clefts between the renal tubules. But even in this respect the 

 process does not present any exclusive feature, for in the parts of the organs in which 

 the tubules are more sparse and the mesenchyme looser the process of mesenchymal 

 proliferation leads to results analogous to those observed in the other regions of the 

 body. 



Figure 14 shows the striking changes undergone by the mesenchyme around a 

 glomerulus and its secreting tubule in the mesonephros of an embryo of 13 days of 

 incubation. This figure is a photograph of a preparation and therefore removes all 

 doubts as to the authenticity of the process as well as to the intensity which it may 



