G. Carl Huber 49 



At the same time, the nuclei take a position in the basal portion of the 

 cells, so that the lumen becomes surrounded by a distinct margin of 

 clear protoplasm. The nuclei also stain less deeply than in earlier stages 

 of development and present a vesicular appearance. The lumen of this 

 portion of the tubule, which, before the cell differentiation is observed, 

 is relatively narrow, becomes prominent. This cell differentiation extends 

 the entire length of the proximal portion and for a variable distance along 

 the descending arm of Henle's loop, as developed at this stage. This cell 

 differentiation is shown in I and J of Fig. 3. In the former is shown 

 about the distal one-half of the proximal convoluted portion of the tubule 

 represented in reconstruction in I of Fig. 4, and its junction with the re- 

 mainder of the tubule, the epithelium of which shows as yet no differentia- 

 tion. In the latter is shown a loop of Henle cut through its entire length 

 about the upper half of the descending limb of which shows an epithelium 

 with clear protoplasm and nuclei with basal position. The proximal 

 convoluted portion of the tubule is further seen in cross section just to the 

 right of the upper end of the descending limb of Henle's loop as seen in 

 the figure; this also shows the characteristic cell differentiation. In the 

 immediate vicinity of the Malpighian corpuscle, where the tubule becomes 

 continuous with the outer layer of Bowman's capsule, this cell differentia- 

 tion is not so pronounced as in other parts of the proximal convoluted 

 portion, nor do the cells of this region attain the same size as in other 

 parts of the tubular segment ; the lumen likewise is not so well developed. 

 Consequently the tubule presents here for a short though a variable 

 distance a smaller diameter than in other parts of the proximal convoluted 

 portion and may now be recognized both by reason of its smaller size and 

 by its structure as what is known as the neck of the uriniferous tubule. 

 At this stage of development, the remaining portions of the uriniferous 

 tubule, the greater part of the loop of Henle, the distal convoluted portion, 

 and the junctional tubules present essentially the same structural appear- 

 ance seen in the earlier stages of development. 



The so-called junctional tubules of the uriniferous tubules elongate 

 as these develop with the growth of the kidney. As has just been stated, 

 a Malpighian corpuscle develops in about the location of its anlage; the 

 proximal and distal convoluted portions of a tubule of which such a 

 corpuscle forms a part, differentiate in its immediate vicinity. As the 

 developing kidney grows in size, the collecting tubules elongate, their 

 ends being always found just under the capsule, and as the junctional 

 tubules are inserted at the peripheral ends of the collecting tubules 

 (the exceptions I shall note) the collecting tubules, as they elongate, carry 

 with them the ends of the junctional tubules; those for the uriniferous 



