CI. Car] Huber . 87 



^•enoral terms it niav he stated, higher up in the lueduna for each suc- 

 cessive generation of tuhujes. the loop of those hitest formed extending 

 only to the Ijoundary zone of the medulla and even for the adult human 

 kidney, remaining entii'ely within the medullary rays of the cortex. 

 The descending limb of the loop is the thinner of the two and is lined 

 by a clear, flattened e})ithelium; the ascending liml). showing the greater 

 diameter, is lined by a cubic epithelium with striated protoplasm; the 

 transition from the thin to the thicker portions of the loop occurs in 

 the tubules reconstructed, at the lower end of the descending limb, at a 

 variable distance from the loop itself, though generally near it. This 

 portion of the descending limb and the loop itself show the same diam- 

 eter and epithelium as the ascending limb. Schweiger-Seidel found the 

 transition from the thin to the thicker epithelium in about an equal 

 portion of the tubules — 1, at the lower end of the descending limb; 2, in 

 the loop itself; 3, at a variable distance from the looj) in the ascending 

 limb. His observations have received very general acceptance. Von Ebner, 

 one of the more recent writers, who has slightly modified Schweiger-Seidel's 

 diagram, states that " the place of thickening is inconstant ; now it lies 

 in the descending limb, now in the loop itself, often and quite regularly 

 for loops which extend deep into the pyramid, in the ascending limb." 

 Hamburger finds for the mouse that the loop itself and the ascending 

 limb are lined by a granular epithelium to a few days before birth ; as, 

 however, the loop grows in length with the increase in length of the 

 papilla, the elongation of the loop is obtained through a growth in 

 length of the descending limb, lined l)y the flattened epithelium, so 

 that the loop is formed by it, and only in the basal portion of the pyra- 

 mid in the boundary zone are there found loops with the dark epithe- 

 lium. In making the statement, based on observations made on my 

 models, that the transition from the thinner to the thicker part of the 

 loop of Henle occurs at the lower end of the descending limb, I do so 

 with some reservation, since the models carry the development of the 

 tubules only to the time of birth. I am aware that such statement is 

 open to the criticism that I have applied to Stoerk's observations relat- 

 ing to the size and structure of the descending limb, namel}^ that they 

 were made on tubules not fully developed. Yet I have observed a 

 number of loops in series of sections of kidneys from half-grown and 

 full-grown cats and rabbits, situated near the apex of the Malpighian 

 pyramid, the epithelial lining of which consisted of cubic cells with 

 granular protoplasm, while I have not found clear evidence of a loop 

 lined with flattened epithelium. Complete reconstructions of a number of 

 iiriniferous tubules from tlu' kidneys of. adult animals are necessary to 



