82 CHRISTIANNA SMITH 
times homogeneous when fixed and stained by Bell’s method, 
were here very definitely composed of granules which stained 
black (fig. 13). These granular filaments starting at the base of 
the cell extended to the edge bordering the lumen, and occupied 
the space between the nucleus and periphery of the cell. They 
ran parallel to each other and their beaded appearance was 
absolutely clear. In most tubules, these granular filaments so 
very distinct in the medulla were not preserved in the cortex. 
This was probably due to imperfect preservation, for in the 
dog’s kidney, which will be described later, they were well 
preserved in the cortex. The conclusion seems justified, there- 
fore, that these homogeneous rods which Policard describes as 
“protoplasmic rods with a lipoid surface covering” never becom- 
ing granular, are lipoids and do become granular filaments which 
stain black with osmic acid when treated with certain fixers, as 
a mixture of potassium dichromate and osmic > acid, with or 
without the application of heat. 
The collecting tubules did not contain lipoids that reduced 
osmic acid. 
Benda’s flwid. Material fixed in Benda’s fluid showed the 
same characteristics as that fixed in potassium dichromate and 
osmic acid. The preservation of the lipoids, as it has been 
stated before, was better in’the bichromate solutions. 
One per cent osmic acid. In a simple solution of osmic acid, 
the same details were evident that were present in the other 
osmic-acid fixers. The rods of the ascending limb of the medul- 
lary loop appear granular upon focusing, but homogeneous when 
in packets. 
The discussion of the tissues fixed in Helly’s fluid, Zenker, and 
Regaud will be omitted in this paper and taken up in another, 
when the effects of different fixers and the lipoids of the kidney 
will be studied in their relation to mitochondria. 
As an example of a very fat kidney with a very black cortex, one 
will be taken from a cat which measured 46 em. occipital crest 
to the base of the tail. This kidney possessed a very yellow 
cortex in the fresh condition and was probably pathological, as 
the liver appeared to be also. 
