The development of the kidney in the rabbit. 285 



any blcinie to Balfour for the slip that he evidently made as an 

 interpreter of Sedgwick's work, one is rather astounded at his accuracy, 

 as an observer. He was able to leave behind so magnificent and 

 lasting a memento of careful observation and of judicious inference, 

 that his statements have been received without perhaps that element 

 of doubt and mistrust which leads often to further questioning: so it 

 is that this mistake has escaped detection, and remains in evidence of 

 the faith that all his fellow workers placed in him. 



Absence of facts in favour of the Balfour Sedgwick view. 



We now turn again to the actual evidence in favour of the view 

 that the epithelium of the tubules arises from the blastema and not 

 from the epithelium of the branched collecting tubules lying in the 

 blastema. The most conclusive evidence would be the presence of 

 isolated masses of cells and their subsequent junction with the tubules, 

 but of this, as we have seen there is no proof. What we have, is 

 an appearance at the tips of the collecting tubules which suggested 

 to the minds of many observers, that the cells of the blastema grad- 

 ually transform themselves into epithelium. They appear to transform 

 themselves gradually into epithelial cells and to arrange themselves so 

 as to lengthen the already existing tubule. In Sedgwick's figures, one 

 of which has been reproduced (fig. 1) the cells of the blastema are 

 shewn as branched cells joining with each other to form a mesh-work. 

 Those that are at the top of the already existing tubule lose this 

 branched character and appear to approximate in character to the 

 short columnar cells of the tubule itself: they are supposed to be 

 differentiating into the epithelium which is to form the tube. This is 

 the evidence adduced by Sedgwick. I would submit however the 

 suggestion that these branching cells of the blastema are already 

 young connective tissue cells, and have themselves differentiated from 

 the more epithelial-like cells which one always finds in the original 

 intermediate cell mass at an earlier stage. No histologist looking at 

 the figure given by Sedgwick would fail to recognise as characteristic 

 embryonic connective tissue these cells which he figures as precursors 

 of the Kidney tubules. We have therefore to suppose, if we accept 



