G. Carl Ilubcr ' 43 



in its elongation it is forced to acquire secondary curvatures. Of these 

 one is fairly constant both as to the time of its appearance and as to its 

 location, appearing soon after the S-stage is reached and involving about 

 the middle of the upper S-curve. The convexity of this curve is often, 

 though not always, directed inwards toward the collecting tubule. Soon 

 after the appearance of this curvature or coincident with it, the region of 

 the junction of the S-middle piece and lower S-curve becomes rounded 

 out into an arched tubule with convexity turned upwards, the lower 

 S-curve contributing to its formation. Hence, as previously stated, not 

 all of the lower S-curve goes to form the bowl-shaped structure. A por- 

 tion of variable length in the region of its junction with the S-middle 

 piece assumes a tubular form and with a portion of the S-middle piece 

 forms the arched tubular segment to which reference is here made. This 

 tubular segment may as a whole be bent forward or backward or may early 

 acquire secondary curvatures. The region of the junction of the upper 

 S-curve with the S-middle piece now forms a distinct loop. This region 

 in the S-stage lies in the sagittal plane of the tubular anlage, as may be 

 seen in D and E of Fig. 3. Coincident with tlie developmental changes 

 above referred to, this portion of the tubular anlage becomes turned on 

 its axis to a variable extent, the arm of the loop formed by the lower por- 

 tion of the upper S-curve being moved backward, so that the plane of 

 the loop now intersects a plane passing as in sagittal section through the 

 portion representing the lower S-curve. The angle formed by these two 

 planes varies in the different anlagen from an acute" angle to nearly a 

 right angle. These changes in form and relative position of the upper 

 S-curve and S-middle piece seen in various degrees of development are 

 shown in the models reproduced in E, F, and G of Fig. 4, D of Fig. 5, 

 and D and E of Fig. 6. E, F, and G of Fig. 3 show the appearance pre- 

 sented by tubular anlagen in these stages when seen in sagittal sections ; 

 E shows a tubular anlage when the tubular portion deviates but slightly 

 from the sagittal plane, the tubule being cut through its entire length, 

 its lumen appearing, however, only here and there, wliile in D the curva- 

 ture of the tubule has progressed to such an extent that only a part of it 

 falls within the plane of a sagittal section passing through the middle of 

 the anlage; this is more marked in G of this figure. A comparison of 

 these figures with E, F, and G of Fig. 4, reconstructions of the respective 

 tubules, will assist in their interpretation. While the above mentioned 

 developmental changes, wJiicli affect the tul)u]ar portion of the anlage, 

 are in progress, changes affecting the lower S-curve are also to be ob- 

 served. These changes consist in an enlargement as a whole of this 

 portion and in a growing upwards of its border, thus deepening its con- 



