(i. ("arl HuIkt . ?5 



instance seen in the tubule of Fig. 13) consist primarily in an elonga- 

 tion of the tubular jDortion, this affecting all its parts, but especially the 

 loop of Henle. The data gained by me concerning the later stages in 

 the development of uriniferous tubules are confined to a large extent to 

 those obtained from observations on older embryos of cat and rabbit, the 

 kidneys of which are of relatively simple type, with only one Malpighian 

 pyramid. The disposition of the collecting tubules and other tubular 

 elements of the medulla of such a kidney is such that both in series of 

 cross and longitudinal sections a certain few will be found in which the 

 plane of section is parallel or very nearly parallel to certain of the col- 

 lecting tubules and loops of Henle. Nearly every such series will show 

 here and there a loop, one or the other arm of which is cut for nearly 

 its entire length, and, by using this as a starting point, I have generally 

 succeeded in tracing out the remainder of the tubule in question. In the 

 human kidney of the later periods of foetal life, the conditions are com- 

 plicated by reason of the fact that the cortex is divided into a number 

 of primary and secondary lobules, and even when one lobule is used for 

 sectioning I have found it quite impossible to orient the block in such a 

 way as to obtain long segments of either of the two arms of the loop of 

 Henle, especially for the upper regions of the medulla. In this region the 

 tubular elements very generally appear in oblique section and are sepa- 

 rated in the kidneys of human embryos of the 8th and 9th months by a 

 relatively small amount of interstitial tissue, which makes the tracing of 

 a single tubule through this region and, as is often necessary, through a 

 long series of sections a matter of difficulty. From a number of partial 

 reconstructions which were made, I am led to believe that the observations 

 made on uriniferous tubules of the older embryos of cat and rabbit (espe- 

 cially the former) may be accepted as presenting the conditions shown 

 by uriniferous tubules of human embryos of the later months of foetal 

 life. My observations on the later stages of development of uriniferous 

 tubules, I shall present by giving a brief description of certain tubules 

 showing these, which have been reconstructed in full and which are here 

 figured. In Fig. 18 are shown two uriniferous tubules, the Malpighian 

 corpuscles of which are situated in about the middle zone of the cortical 

 portion of the kidneys from which they were reconstructed and may, 

 therefore, be taken as representative of tubules which were differentiated 

 after the first formed tubules of the respective kidneys. Tubule A is 

 from the kidney of a cat embryo obtained a few days before birth. The 

 length of tliis tubule is 4.1 mm., of which 1.75 mm. falls to the proxi- 

 mal convolutoil portion, 1.G5 mm. to the entire loop of Henle (the measure- 

 ment here beginning and ending with the level of the lower border of the 



