46 Development and Shape of Uriniferous Tubules 



developing kidney, as shown in F of Pig. G, or grows downward from 

 the beginning and in doing so appears to push aside the anlage of Bow- 

 man's capsule, which in being pushed aside is turned on its axis so that 

 its posterior border attains a higher level than its anterior border. This 

 is very well shown in G of Fig. 6, also, though not quite so clearly, in E 

 of Fig. 5. While the loop of Henle is thus developing, the portion which 

 is to form its ascending or distal limb remains in close proximity and 

 on its inner side to the developing Bowman's capsule, after crossing the 

 place where this is joined to the tubular portion which forms the anlage 

 for the proximal convoluted tubule, the descending or proximal limb 

 of the loop lying to the inner side or slightly in front of the distal limb. 

 The two arms of the loop are generally, from the time when they may 

 be clearly recognized, parallel or nearly parallel; now and then the de- 

 scending limb may be twisted slightly over the ascending limb, as in 

 F of Fig. 5. In the models here reproduced, which show the early 

 stages in the development of the loop of Henle, it will be observed that 

 the loop of Henle grows down in front of the anlage of the Malpighian 

 corpuscle or in front of the tubular segment attached to it. This may 

 be accepted as a general law and holds good also for the tubules develop- 

 ing to the left of the collecting tubules, the ones shown in the figure all 

 developing to the right of the collecting tubules. This statement I 

 shall desire to amplify somewhat in discussing more fully the relations 

 of the developing uriniferous tubule to the collecting tubules when I 

 shall also consider Stoerk's observations on this point. Coincident- with 

 the formation of the definite anlage of the loop of Henle, as here given, 

 the portions of the tubule which I have designated as destined to form 

 the proximal and distal convoluted portions also increase in length, 

 especially the former, which grows upward and acquires two or three 

 secondary curves. It begins at the developing Malpighian corpuscle and 

 extends upward behind the anlage of the loop of Henle, then arches for- 

 ward between the collecting tubule and the other parts of the respective 

 uriniferous tubule, then passes downward to become continuous with the 

 descending limb of Henle's loop. Just before tlie loop is reached, it 

 often shows a distinct flexure, which is fairly constant and character- 

 istic, appearing also in older stages. The curvature which forms the 

 anlage of the distal convoluted portion does not grow in length as rap- 

 idly as that part which forms the proximal convoluted portion. The 

 curvature becomes more pronounced as development proceeds and may 

 obtain one or more secondary bends, or may show in much older stages 

 than here discussed a single bend. It may here be added that there 

 is observed a considerable variation as to the relative lensffli of this 



