MYELOID METAPLASIA OF THE EMBRYONIC MESENCHYME. 21 



to be found in literature concerning the fate of the mesenchymal or connective- 

 tissue cells present at that time in the mesonephros. Their active proliferation 

 and differentiation, manifested after grafts at a time when the epithelial tissue 

 begins its normal involution, is a direct proof of the great synthetic power belong- 

 ing to the mesenchymal cells of the mesonephros. In some way the gradual low- 

 ering of the metabolic processes in the renal epithelium, with ensuing death, seems 

 to be a rather favorable agent in bringing about an intensification of the prolifer- 

 ative processes in the mesenchymal cells and in the granuloblasts. 



Though of a common origin, the mesenchyme of the mesonephros and its renal 

 epithelium have diverged in their development. The removal of the substances 

 excreted by the epithelium into the lumina of the renal tubules secures for the 

 mesenchyme a protection against contact with possibly injurious substances and 

 the mesenchyme in the mesonephros of a 7 to 10 days' embryo retains its prolifera- 

 tive and differentiative ability, as seen from the actual exercise of its granuloblasts 

 potentiality. This potentiality, as follows from an analysis of the changes occurring 

 after grafts in the mesenchyme of other organs, is a common property shared 

 equally by all of the mesenchyme found at this time in the organism. 



Mesenchyme of the metanephros. — Unlike the mesonephros, at the time when 

 the graft of adult splenic tissue was made on the allantois of the embryo, the meta- 

 nephros is the center of most intense organizing and proliferative processes; new 

 renal tubules are developing, and those already present develop further into their 

 different parts. The growth and development of the specific renal tissue seems to 

 be little if at all affected by the common stimulation of the mesenchyme. The 

 differentiation of definite parts of the nephrogenic cord appears as a specific process, 

 at least under the conditions of the present experiments, as well as in typical develop- 

 ment. The development of the metanephros follows its own course and attains 

 results identical with those under normal conditions. The mesenchyme, however, 

 which at this time is intimately intermingled with the nephrogenic tissue, detaches 

 numerous cells just as in the other parts of the embryo already examined, and 2 or 

 3 days after the grafting was done — that is, on the eleventh day of incubation- 

 large basophilic hemoblasts may be seen in the spaces between the renal tubules 

 and the capillaries and around the newly developing renal units. At this early 

 stage the circulating blood in the vessels of the metanephros does not show any 

 marked changes in its cellular elements and contains only more or less differentiated 

 blood cells. There is not the least doubt, therefore, that these hemoblasts develop 

 from the loose mesenchyme more or less abundant at this time in the organ and 

 are not cells which have immigrated from the vessels. 



It is quite remarkable that in the regions of the developing metanephros in 

 which the tubules are seen to originate, and which at this time consist of morpho- 

 logically undifferentiated stellate and branched cells, comparatively few of the 

 latter transform into mobile cells. The great bulk of the nephrogenic tissue organizes 

 itself into the specific renal epithelium. Whether or not the nephrogenic tissue 

 develops from two substantially different sources (for renal epithelium and stroma) 

 under the aspect of a morphologically identical structure I can not at present say 



