EXCRETION 459 



was blue in the medulla also. The ' rodded ' epithelium was 

 filled with blue granules as before (Fig. 174). 



(i) Shows that the epithelium is capable of excreting some 

 substances at least. (2) Appears to show that when the blood- 

 pressure is normal water is poured out from some part of the 

 tubule, and washes the pigment separated by the ' rodded ' 

 epithelium down towards the papillae. (3) Suggests that it is 

 through the glomeruli that most of the water passes. For 

 cauterization has not destroyed the power of the epithelium 

 to excrete pigment, and therefore, presumably, would not have 

 destroyed its power to excrete water if it possessed this power 

 in any great degree ; and the glomeruli and their capsules are 

 the only other part of the renal mechanism which can have 

 been affected. The fact that in birds and serpents, whose urine 

 is solid or semi-solid, the glomeruli are smaller than in mammals 

 is corroborative evidence that the glomeruli have to do with 

 the excretion of water. 



When pigments are injected into the dorsal lymph-sac of a 

 frog without interference with the renal circulation, they are 

 found plentifully in the lumen of the convoluted tubules, and 

 also in the epithelial cells lining them. The suggestion has been 

 made that the pigments have been absorbed by the cells from 

 the lumen and not excreted by them into it. And certainly 

 pigments soluble in the cytoplasm or in the substances that 

 form the envelopes of cells, and therefore capable, like methylene 

 blue, of staining them during life, might be taken up by the 

 renal epithelium if excreted into the tubules by the glomeruli, 

 and might cause staining of them, particularly, of course, of the 

 free ends of the cells next the lumen. But this suggestion is 

 inadmissible since on injection of the same pigments after ligation 

 of the renal portal vein the convoluted tubules contain little or 

 no pigment in their lumen . And when the urinary flow is stopped 

 on one side in mammals by temporary compression of the renal 

 artery the corresponding kidney takes up fully as much carmine 

 as its fellow (Carter). There is no doubt that not only pigments 

 capable of ' vital staining,' like methylene blue, but also pigments 

 which do not stain living cells, are taken up from the blood (or 

 lymph) by the epithelial cells, and, lying in vacuoles in their 

 cytoplasm, are transported towards the lumen, and there ex- 

 truded. It is not the solubility of the pigments in lipoids, and 

 therefore their solubility in the supposed lipoid envelope of the 

 cells, which determines whether they shall be excreted. The 

 degree in which they are capable of being presented to the cells 

 in non-colloid solution appears to some extent to be a determining 

 factor. The pigments not taken up are highly colloidal (Gurwitsch, 

 Hober). Shafer has recently confirmed Heidenhain's state- 



