1394] The Action of Leucocytes Toward Foreign Substances. 319 
the similarity in colour and from the necessary obscuring of 
structural parts. Caustic potash destroys melanin, but boiling 
is required and that of necessity injures the tissues. Ether, 
alcohol, acids and strong alkalies will also remove the colour, 
but the last two destroy the tissues and the first two decolorize 
so slowly as to be practically useless. By means of hydrogen 
dioxide the most successful results were obtained. The sec- 
tions when cut and fastened to the slide were put in a vial of a 
2% solution of the liquid. In from 6-48 hours, depending on 
the amount of pigment present, the color is reduced from black 
toa pale yellow without any attendant injury to the tissues. 
The process of decolourization is materially hastenéd by plac- 
ing the vial containing the liquid and tissue in the strong sun- 
light and if desired all traces of the pigment can be removed. 
Practically it was found to be a great advantage to leave 
sufficient colour to mark the position of the pigment-bearing 
cells. By this method the black ingested leucocytes were easily 
distinguished wherever they oceurred, and no chance for con- 
fusion remained. : 
Serial sections were made of the following parts: the spleen, 
kidney, ureters, liver, lung, stomach, muscle and skin. In all 
these parts ingested cells were present, but the positions and 
relations differed somewhat with the different organs. In the 
kidney (Pl. VI) carbon-laden leucocytes were in the blood 
capillaries, in the glomeruli, in the lymph spaces surrounding 
the capsules of the glomeruli, in the urinary tubules and in 
the nephrostomes. These latter parts are peculiar structures 
present in the amphibian kidney and are marks of a much 
more primitive form of that organ than exists in mammals. 
They consist of small ciliated funnels opening on the ventral 
surface of the kidney directly into the abdominal cavity. A 
small tube then unites these funnels with the urinary tubule 
arising from the glomeruli. The ingested leucocytes were 1n 
these funnels and by a series of sections they could be found 
to pass down the tube and into the urinary tubule. No doubt 
the number of leucocytes that pass from the blood circulation 
into the tubules is largely increased by additions from this 
source. No signs of ingested leucocytes in other than these 
