G. Carl Huber . 23 



nuclei which stain relatively deeply, the layer being in contact with epi- 

 thelial cells which form .the wall of the ampullar enlargement. This 

 nephrogenic tissue is therefore found at the periphery of the developing 

 kidney immediately under the capsule. From this tissue, with each 

 successive division of the ampullar enlargements of the end branches of 

 the collecting tubules, new generations of renal vesicles are differentiated 

 in a manner similar to that previously described for the first generation 

 of renal vesicles. While new renal vesicles are thus differentiating 

 about the ends of the terminal branches of the collecting tubules, those 

 previously formed are developing into uriniferous tubules. Those first 

 formed show the greatest degree of development, the various generations 

 of renal vesicles undergoing essentially the same metamorphosis in devel- 

 oping into uriniferous tubules. Beginning with a relatively early stage 

 in the development of the kidney through the entire period when new 

 tubules are forming, there may therefore be observed a peripheral zone 

 containing the end branches of the collecting tubules showing ampullar 

 enlargements surrounded by nephrogenic tissue, forming and formed 

 renal vesicles, and of these others which show the earlier stages of 

 development leading to the formation of uriniferous tubules. This sub- 

 capsular zone stains somewhat more deeply than other portions of the 

 developing kidney and may therefore be recognized with the naked eye. 

 It surrounds the entire kidney except the place where the ureter enters. 

 It has repeatedly been recognized as the zone where the earliest stages 

 of the developing uriniferous tubules are to be found and is described by 

 Hamburger and Stoerk as the neogenic zone ("die neogene Zone" — 

 Hamburger). In this way new generations of uriniferous tubules de- 

 velop outside of those previously formed, the latter thus coming to lie 

 deeper down in the parenchyma of the kidney and showing according to 

 their age more or less advanced stages of development. The period when 

 the new formation of renal vesicles ceases varies somewhat for different 

 animals. In the rabbit, I have observed their formation as late as the 

 first week after birth, although the number of newly forming renal ves- 

 icles is for the later period of embryonic life relatively small. In the 

 cat, newly forming renal vesicles were observed in a kidney removed 

 from an embryo a short time before birth. Herring states that no more 

 tubules are formed in the human kidney after the 8th month of foetal 

 life. Stoerk, who has considered this question quite fully, agrees with 

 Herring. Toldt, on the other hand, states that new Malpighian cor- 

 puscles form in dog and man in the entire peripheral part of the kidney 

 for 8 to 10 days after birth. My own human material is too limited to 

 he of any value in determining this question. 



