IRON COMPOUNDS IN ANIMAL AND VEGETABLE CELLS. 237 
to the difficulty experienced in getting examples of such forms 
in the numbers required, it was impossible to make a fully 
satisfactory, systematic investigation of their iron-holding 
character. On the other hand, examples of the genera named 
could be obtained at all times in abundance, and I regard the 
opportunities thus presented as compensating in some measure 
for the limited range of genera studied. 
One of the difficulties encountered in attempting to study 
the distribution of iron-compounds in Protoza is the fact that 
many of the motile forms, and some also of those which are 
sessile or attached, have in their cytoplasm inorganic com- 
pounds of iron, in great part, if not wholly, derived from the 
food matters ingested, and when such organisms, after being 
hardened in alcohol, are treated with the glycerine and sulphide 
mixture, they give at once a deep reaction for iron which, in 
many cases, obscures other details in the cytoplasm and 
nucleus. When, moreover, attempts are made with acid 
alcohols, and especially Bunge’s fluid, to remove the inorganic 
iron, the conditions under which the experiments are made 
enable the reagent to liberate the “‘ masked ” iron at the same 
time, in which case the liberated portion becomes indistinguish- 
able from that present previously in an inorganic form. To 
avoid such difficulties it is necessary to select forms in which 
the amount of inorganic iron is small or infinitesimal, and by 
determining the amount of the reaction obtained during the 
first ten minutes after the application of the glycerine and 
sulphide mixture, one may thus prevent confusion arising from 
the study of results obtained by the more prolonged applica- 
tion of the reagent. Such forms may be found in the genera 
above named, and one may, by attention to the character of the 
medium of the organisms, without any difficulty secure such 
examples as offer the most favourable conditions for investi- 
gating the distribution in them of the assimilated iron. The 
specimens of Hpistylis, for example, which were used by me 
for this purpose, were obtained from a colonial form attached 
to the sides and limbs of the common crayfish, and their cyto- 
plasm gave no immediate reaction for iron. Examples of 
