240 A. B. MACALLUM. 
glycerine and sulphide mixture. In examples absolutely free 
from inorganic iron compounds the reaction in the cytoplasm 
and nucleus was as marked as that represented in the figure. 
The method is, of course, open to the objection that it may 
permit a diffusion of the liberated iron from the nucleus to the 
cytoplasm, but that the latter contains assimilated iron is 
shown by prolonged treatment with the warm glycerine and 
sulphide reagent. 
In examples of different species of Paramcecium, the 
cytoplasm, which gave no reaction for inorganic iron, mani- 
fested with the warm glycerine and sulphide reagent after ten 
days a reaction as distinct as that obtained under similar 
conditions in the cytoplasm of Stentor, Vorticella, and 
Epistylis. These organisms were the only ones in which 
the micro-nucleus was revealed by the iron reaction, and the 
latter appeared to me to develop more slowly than that in the 
macro-nucleus; but the explanation for this may be that the 
large quantity of chromatin in the latter renders a reaction 
of any degree of intensity obtaining in it much more promi- 
nent than a reaction of a similar intensity would appear in 
the micro-nucleus. In both the reaction was almost wholly 
confined to the granules and fibrillar elements. 
All the forms of Protozoa studied illustrated the fact so 
prominently indicated in the organisms referred to above, that 
an assimilated compound of iron is a constant element in 
their cytoplasm. It is probable that this compound belongs 
to the chromatin class, for the cytoplasm in Protozoan 
organisms generally stains much more readily, and holds the 
dyes more tenaciously, than the cytoplasm in higher organisms 
does. In support of this may be urged other facts. I 
pointed out, when dealing with the relations of assimilated 
iron compounds to the ferment-forming cells in Vertebrates, 
that the substance which elaborates the ferment, or out of 
which it is prepared, contains iron and acts towards staining 
reagents like chromatin. Digestion in Protozoa is, in all 
probability, effected by ferments derived, as in higher forms, 
from the cytoplasm, and it is only reasonable to suppose that 
