90 CHRISTIANNA SMITH 
as a source of lipoids in the cells is discussed by Cowdry (16), 
who thinks that it would require no great stretch of imagination 
to believe that this transformation could take place. The 
intracellular origin of lipoids does not, however, exclude an 
extracellular origin also, as shown by Bell in his experiments 
with rats (Bell, 714-15). 
Furthermore, the presence of lipoids in renal cells, either 
masked or free, the presence of large amounts of lipoids in cells 
of the proximal convoluted tubule where it is generally conceded 
either secretion or absorption takes place, the characteristic 
occurrence of typical lipoid formations (large droplets in the 
medullary limb of the proximal convoluted tubule in cat, and the 
rods in the ascending limb of the medullary loop), do not indicate 
that lipoids in kidney cells are merely passive structures, nor 
simply droplets formed by the coalescence of finer granules in 
an emulsion, but that they may be a direct expression of activity 
in the cell economy. Bullard (12, ’16) and Bell (11) have con- 
cluded that lipoids in normal muscle cells are a reserve food 
supply. In discussing the réle of fats (lipoids) in vital phenom- 
ena, Leathes (’10) says that they are most conspicuous as a 
reserve fund of fuel for growing and working cells, but that in 
virtue of their general chemical inertness are capable of being 
put to many uses in the organization of plants and animals, for 
instance, being essential to the cohesion and physical constitution 
of the protoplasm. In connection with the use of fat as fuel 
for working and growing cells, Hatai (’15) finds that a lipoid-free 
ration diminishes the normal rate of growth of the body in albino 
rats. Leathes also says that the unsaturated fats found in the 
cells of the body are broken down by successive oxidations until 
they are completely burnt to oxygen and water. Imrie, in his 
discussion of the fatty changes in the liver, heart, and kidney, 
says that the lipoids are oxidized in them to supply energy. In 
other words, the lipoids are reducing, and their probable relation 
to the reducing power of cytoplasm is discussed by Kingsbury 
(12) in his paper on cytoplasmic fixation. In this paper it is 
pointed out that as early as 1885 Ehrlich called attention to the 
