186 JOHN LEWIS BREMER 



presumably present in the glomerulus, but limited to the thicker, 

 still granular portions of the cells. 



I have considered the glomerulus thus minutely, and offer a 

 drawing of a portion of one in normal activity (fig. 1) because 

 of the great lack of accurate descriptions and figures in the 

 text-books. This is the more remarkable in contrast with the 

 elaborate care with which the tubules almost uniformly are 

 described. 



The presence of thin plates of epithelium covering the capil- 

 lary blood vessels is, then, the anatomical indication of that 

 part of the excretory function which takes place in the glomeruli, 

 either of the mesonephros or of the kidney. But it is not a sure 

 sign that excretion is actually taking place at the spot where it 

 is found, and for two reasons. The first is that diffusion or osmo- 

 sis is dependent on the proper pressure on either side of the 

 membrane, and on the proper chemical constitution of the 

 fluid on the two sides; failing these two requisites the plates 

 may be present but inactive. The second reason is of a differ- 

 ent kind, namely that another physiological process, the ex- 

 change of oxygen for carbon dioxide and water, also takes place 

 mainly through the medium of thin plates overlying capillaries. 

 In the ' breathing epithelium' of the lungs and of the gills of the 

 different types of vertebrates there is again found the modifica- 

 tion of the epithelial layer, originally columnar, to a succession 

 of thin plates, covering capillaries, and nucleated masses of pro- 

 toplasm either projecting or imbedded in the meshes of the cap- 

 illary network. Here again is a provision for both physical 

 diffusion and physiological active secretion or excretion. As 

 far as the simple diffusion is concerned, there is no physical law 

 to prevent the passage of certain urinary constituents, oxy- 

 gen, and carbon dioxide all at the same time, even in opposite 

 directions, through the same membrane, if the conditions on 

 the two sides of the membrane are favorable. In ordinary breath- 

 ing this exchange in two directions is manifest. In other words, 

 in seeking to establish the fact of a urinary excretion from the 

 placenta by showing there the thin plates in their proper rela- 

 tion to the capillaries, it is necessary as far as possible to elimi- 



