G. Caii liuber 39 



The account of the further development of the lower S-curve as given by Stoerk I 

 shall give in his own words, as it is particularly with this portion that I desire to take 

 exception. He states (pa:;e 'J'JD : " Die Mnscliol niinnit ciiiorseits im Sinne der Flache 

 an Grosse zu, andererseits gewinnt sie an Wolbung, sodass sie zuniichst der Form 

 einer ein wenig verzerrtcn halben Kugelschale zustrebt (s. Pig. 3, 4 und 5) ; diese 

 umwolbt die Uobergangstelle voni Mittelstiick in die obere S-Hlilfte und wird von dieser 

 Kaniilchenpartie fast bis zuni Kontakt erfiillt ; allmilhlich wird dann auch das S-Mittel- 

 stiick zur Kugelsclinlenbildung lierangezogen und geht in diese auf (Modell B), so 

 dass nunmehr der obere S-Bogen iibrig bleibt, der, aus der Kugelschale aufsteigende, von 

 deren Innenseite durch eben .ienen scbmalen von Zwischengewebe erfiillten Raum 

 getrennt wird, der sich, wie erwahnt, im Schnittbilde als abwiirts gekriimmter enger 

 Spalt priisentiert." 



This double-walled structure, developing from the lower S-curve and 

 the S-middle piece, with the enclosed mesenchyme, develops into Bow- 

 man's capsule and glomerulus. While these are developing, and very 

 soon after the stage is reached to which reference is made in the quota- 

 tion, the upper S-curve elongates and becomes flexed and for a time 

 again has the shape of an S. This Stoerk designates "secondary S," 

 in contradistinction to the primary S, of which, after Bowman's capsule 

 is formed, there remains only the upper S-curve. In reality, as he states, 

 there is no primary S, this being seen only in sections of this structure 

 at this stage. He further states that the want of recognition of these 

 two S-stages and their time relation, on the part of former observers, 

 accounts for the contradiction which is found between his view and the 

 views expressed by other observers as to the genesis of the different parts 

 of the uriniferous tubules, with reference to the different parts of the 

 S-figure. In discussing these observations of Stoerk, I desire in the 

 first place to call attention to the fact that, while the various anlagen of 

 uriniferous tubules from a time when they may be differentiated from 

 the renal vesicles to a time when they present a distinct S-shape show 

 at different stages of their development certain typic forms which are 

 again and again met with, they after all vary in certain details both as 

 to size and form and also as to the relative time at which certain changes 

 in form occur. This is true not only when comparing tubular anlagen 

 obtained from different species of animals, but also when comparing such 

 obtained from embryos of the same species. With some reservation, the 

 statement may be made that tubular anlagen which develop from the 

 first few generations of renal vesicles are larger and present a slightly 

 different form from those which develop from later generations. In 

 the earlier stages of the development of the kidney, the branched collect- 

 ing tubules are relatively far apart and the renal vesicles and tubular 

 anlagen which develop in connection with any one system of end-branches 

 are separated by a relatively large amount of interstitial tissue. This 

 enables the tubular anlagen which develop at this period, when these are 

 considered as a whole, to assume a more rounded shape than can the 

 tubular anlagen which develop in the neogenic and subneogenic zones in 

 the later periods of kidney development at a time when the tubular struc- 



