286 J- B. Haycraft, 



the Sedgwick and Balfour view, that the cells of the blastema first 

 become young connective tissue, and then take on another line of 

 development, and are transformed into epithelium; an alternation of 

 generation about which one might well feel some degree of scepticism. 

 It is not however necessary to urge this objection to their views, for 

 the appearances described and figured by Sedgwick are very faithful 

 representations of what may be seen in all well prepared sections of 

 embryonic Kidneys, and they are nothing more or less than oblique 

 sections through tubules, not at their extremities, but through bends 

 in their course. In my own sections of the rabbit's Kidney at an 

 early age, the tubules bend about in the blastema, they are some 

 times cut transversely but more often obliquely. At the cut end the 

 appearance described by Sedgwick is always seen, and it is indeed 

 very deceptive. The basement membrane disappears and one gets 

 what appear to be badly formed epithelium which quite at the end 

 of the tubule looks more and more like the connective tissue of the 

 blastema. With a good set of sections, carefully cut in series, it is 

 however impossible to suifer deception upon this point once suspicion 

 has been aroused: the next and the succeeding sections shew that the 

 tubule is not cut at its extremity but that it can be traced on into 

 other parts of the blastema; the basement membrane, apparently lost, 

 is seen as soon as the plane of section runs through the cavity of 

 the tubule; while at the position occupied by the cells supposed to be 

 changing into epithelium, fully formed epithelium can be seen. In 

 fig. 2 three sections of the same tubule fi'om the Kidney of a 16 days 

 rabbit are represented: they have been placed in vertical series and 

 corresponding parts are therefore above each other. In a at the 

 spot marked X the cavity of the tubule appears to come to an end, 

 and so does the basement membrane, while there is a mass of epi- 

 thelial looking cells crowded together and forming the top of the 

 tubule. This appearance is precisely similar to the one which led 

 Sedgwick and Balfour to believe that the Kidney blastema is con- 

 verted into the epithelium of the tubule, yet the succeeding sections 

 of the same tubule h and c shew that instead of the appearance at 



