186 A. B. MACCALLUM. 
preparations represented is that of the iron reaction illustrated. 
This is partly due to the fact that in one preparation the 
ammonium hydrogen sulphide has not liberated all the iron of 
the chromatin, but partly also to the fact that the reagent has 
caused the delicate chromatin elements to become swollen, 
thereby rendering the iron reaction more diffuse and less 
marked. The effect on the cytoplasm is not less striking. It 
is, however, chiefly with concentrated solutions of ammonium 
hydrogen sulphide that preparations of animal nuclei exhibit 
this phenomenon. Solutions of the reagent made from am- 
monia of 0:96 specific gravity do not as readily produce this 
change, and in many cases none at all may be shown. When 
the reagent is fresh the reaction is quickly obtained, sometimes 
in two or three days, and then no swelling of the nuclear net- 
work occurs; but when it is not fresh, or when it gives an 
odour of ammonia, the reaction is slowly obtained, and the 
prolonged application necessary in order to bring out this 
result, aided perhaps by the ammonia, causes a swelling of the 
chromatic elements. 
The slowness with which the reaction comes out is not 
wholly a disadvantage, for by this means one may determine 
whether the iron demonstrated is derived from other than 
inorganic or albuminate compounds. With the exception of 
hemoglobin, hematin, and the compound found in yolk- 
spherules, the organic combinations in which the iron is 
“masked” are affected very slowly by ammonium hydrogen 
sulphide, and only when heat is applied ; whereas the reaction 
comes out at once, or after a few minutes at the longest, and 
without heat, in the case of inorganic and albuminate com- 
pounds. The distinction between these and the ‘ masked” 
compounds is, therefore, very marked. In one of the excep- 
tions mentioned the distinction is not so clear, for when 
ammonium hydrogen sulphide is added to the fresh yolk of 
hen’s egg it gives a greenish reaction at once, but when the 
yolk is hardened with alcohol or with heat the reagent gives 
this result only after several days’ application at 50—60° C, 
On the other hand, the yolk-spherules in Amphibia (Necturus 
