G. Carl Kuber . 73 



o])iiunit of urinifcrous tiibnles, the loop of Henle clon^utos at a relatively 

 late period, developing and differentiating as do the loops of other tubules. 

 I agree, therefore, with Minot when he states : " Some authors liave 

 maintained that there is an atropliy of some of the tubules of the fcetal 

 kidney, but I agree with Golgi in believing that of this there is no valid 

 evidence." In offering an explanation for the difference in the develop- 

 ment of the first generation of uriniferous tubules, when these are com- 

 pared with those which develop later, which difference may be character- 

 ized as consisting of a relatively early development of the Malpighian 

 corpuscles and the proximal convoluted portions and a relatively late 

 development of the loop of Henle of the former when compared with the 

 latter, attention may be called primarily to the simple mechanical con- 

 dition which prevents' an elongation of the loop of Henle of the first 

 formed tubules. The Malpighian corpuscle and tubular portion which 

 are first formed develop near the developing pelvis of the kidney, the 

 former being separated from the latter by a relatively narrow zone 

 of mesenchyme. As the tubules proceed in their development, their 

 loops of Henle soon reach the epithelium of the growing pelvis and the 

 denser mesenchyme immediately surrounding it. The loop of Henle of 

 tubules C and D of Fig. 14 and A and B of Fig. 16 reach to the pelvis 

 of the respective kidneys. The ends of such loops are often seen bent 

 to one side or the other, as though attempting to avoid the obstruction 

 which prevents their further elongation, and they are often slightly 

 folded and twisted, the end of the loops being bent so as to project 

 toward the periphery of the kidney, as for instance in tubule .1 of Fig. 16. 

 Sections of developing kidneys of pig embryos of about 2.5 to 5 cm. 

 length (the oldest stage available for me) show this in a very characteris- 

 tic manner. In the series of sections from which the model shown in 

 C of Fig. 17 was made, the Malpighian corpuscle was separated from the 

 pelvis of the kidney by a distance which is about two-thirds the length 

 of the diameter of the corpuscle; a large collecting tubule which opens 

 into the pelvis by a funnel-shaped expansion is nearly in contact with 

 the coiled tubular segment designated in the figure as the descending 

 limb of the loop. There is obviously here no opportunity for the forma- 

 tion and the elongation of a loop with relatively straight and nearly par- 

 allel limbs. The fact that so little mesenchyme separates the tubules 

 and the ]\Ialpighian corpuscles from the pelvis of the kidney, and the 

 shortness of the loop of Henle at a time when the proximal convoluted 

 portions are well deve]o])ed, also the fact that the loops of Henle, so far 

 as developed, are not present in the form of straight tubules, but as coiled 

 tubules, so that only cross or oblique sections of them are obtained, give 



