78 Devt'lopiiiont and Shape of Uriniferous Tubules 



only a small amount of interstitial tissue; its Malpighian corpuscle is 

 situated in the deepest portion of the cortex, just above its junction 

 with the developing medulla ; the prominent coils of the proximal con- 

 voluted portion have a more horizontal position and are, therefore, 

 spread out more in a lateral direction than would be true for tubules 

 which develop later, in which the prominent coils of the proximal con- 

 voluted portion are more prone to grow in a perpendicular direction, 

 toward the cortex. As I have seen no evidence of the atrophy of the 

 first formed tubules — so-called atypical tubules — with relatively short 

 loops of Henle — it is assumed that this loop elongates as the medulla 

 develops. The distal convoluted portion of this tubule lies upon— in 

 front of, as shown in the figure — the coil complex formed by the proxi- 

 mal convoluted portion, the course of which may be ascertained by a 

 study of the key A'. The descending limb of the loop is lined almost 

 throughout by a flattened epithelium and presents a comparatively 

 small diameter. The end segment is relatively short, forming only a 

 small part of the limb. The loop itself and the distal limb have a diam- 

 eter which is just about three times that of the greater part of the 

 descending limb. The sharp bend shown by the ascending limb, just 

 before the Malpighian corpuscle is reached, is due to the fact that a 

 relatively large arterial branch occupies the space just beneath the 

 corpuscle, the distal limb arching partly over this to reach the vicinity 

 of the corpuscle. For this reason also the upper end of the proximal 

 limb is separated by a greater distance from the Malpighian corpuscle 

 than may be considered typical. With these exceptions, this tubule 

 may be regarded as presenting in a very characteristic manner the shape 

 and arrangement of the different parts of a uriniferous tubule, the Mal- 

 pighian corpuscle of which is situated in the deepest portion of the cor- 

 tex, thus of the first few generations of uriniferous tubules. Tubule B 

 of Fig. 19 represents one of the most fully developed tubules of the kid- 

 ney of a rabbit killed the first day after birth. Three exceedingly for- 

 tunate sections of the series of cross sections 5 fi thick, into which one 

 of the kidneys was cut, contained nearly the entire length of the proxi- 

 mal limb of its loop. The model of this tubule measures from the tip 

 of the loop to where it ends in the collecting tubule four feet and four 

 inches. On completing the reconstruction, the tubule proved to be one 

 presenting a not quite typical arrangement of the coils of the proximal 

 convoluted portion, these forming a configuration which is too open 

 and too much in one plane. The cause of this is not readily made out 

 at the stage of development here presented. The tubule shown in 

 of Fig. 14 presents almost the same relations of its parts, and for that 



