64 DuvelopiiR'iit and Shape of Uriniferous Tubules 



tubule figured as seen in section, as is also apparent in references made to 

 it in the text, the clear epithelium found in the proximal convoluted por- 

 tion extends for about half the length of the descending or proximal arm 

 of Henle's loop. This portion of the tubule has also a greater diameter 

 than is shown by the greater part of the remainder of the tubule figured 

 by him, the latter part being lined by the darker epithelium. The tubule 

 shown in Fig. 18 of his article presents about the same stage of develop- 

 ment and cellular differentiation as that shown under J of Fig. 3 of this 

 article, and this tubule was selected as showing a stage of development 

 just prior to that in which the epithelium of the proximal and distal 

 arms of Henle's loop shows the specific differentiation described for them. 

 In A and B of Fig. 13, in which is represented a tubule showing a stage in 

 which the epithelium of all its parts shows characteristic differentiation 

 and represents a stage a little more advanced than the oldest stage figured 

 by Stoerk, it may be clearly seen that that portion of the descending limb 

 of Henle's loop which in J of Fig. 3 or in Fig. 18 (Stoerk's) shows the 

 darker epithelium, differentiates as the loop elongates, into the flattened 

 epithelium characteristic of the greater part of the descending limb of 

 Henle's loop. Stoerk, in formulating his conclusions relating to the 

 shape and structure of the descending and ascending limbs of Henle's 

 loop makes use of data gained from a study of uriniferous tubules 

 representing stages of development in which the epithelial differentia- 

 tion is not as yet complete in all parts of the tubules, notably in the 

 descending and ascending limbs of Henle's loop. It may readily be seen 

 how from such insufficient data, he would be led to the conclusions 

 drawn. If the loop of Henle in either of the tubules shown in J of 

 Fig. 3 or in Stoerk's Fig. 18 were drawn out to form a long loop of 

 Henle, retaining the structure given them in the figures, the result would 

 be a tubule in which the descending limb of the loop would be lined by 

 a clear epithelium to near its end, one which would show a greater 

 diameter than the ascending limb, which would be lined by a darker 

 epithelium. This is what he has done and was thus led to the error he 

 has committed. That he is not justified in assuming that the uriniferous 

 tubules presenting the stages represented in his Figure 18 and model L 

 show the structure and cellular differentiation of fully developed urinifer- 

 ous tubules, may be seen, I believe, in A and B of Fig. 13 and is also 

 shown by my other models showing more advanced stages of development. 

 It may, therefore, again be stated that reconstructions of proper stages in 

 the development of uriniferous tubules show that the descending limb of 

 Henle's loop is lined by a flattened epithelium and presents a smaller 

 diameter than the ascending limb, which is lined by a cubical or short 



