206 A. B. MACALLUM. 
illustrating the Prussian blue reaction, be stained also with 
safranin, which, when carefully employed, affects only the 
chromatin, it will be observed that all the elements coloured 
by the safranin exhibit the blue reaction also, the combina- 
tion of the red and the blue giving to the chromatin a colour 
of a violet shade (figs. 46 and 48). 
Tt is not, however, the chromatin alone in the animal nucleus 
that possesses assimilated iron, for one sees in sections exhibit- 
ing the Prussian blue reaction, but more readily in those which 
have been also stained with safranin, that nucleolar elements 
possess a light blue colour (figs. 45 and 48 a). Some diffi- 
culty is experienced in observing this under ordinary condi- 
tions, but this is overcome, when homogeneous immersion 
apochromatic objectives are employed, by withdrawing the 
diaphragm of the Abbé condenser, the great amount of light 
thus transmitted causing all the blue parts to appear with 
remarkable distinctness, and amongst these the nucleolar bodies 
coloured light blue, while all the other elements are rendered 
indistinct or invisible. When, however, safranin has also 
been employed to stain such preparations, the chromatin 
absorbs it but the nucleolar elements are absolutely unaffected 
by it, and they thus stand out in marked contrast with the 
other structures. Such nucleolar bodies take but a faint stain 
with hematoxylin, a fact which, considered in connection with 
the result of the employment of safranin, would seem to 
demonstrate that they are not essentially formed of what the 
cytologist comprehends under the term chromatin. The 
number of these in a nucleus varies, and the shape and size of 
each are not constant, while not unfrequently the central 
portion appears free from iron, the outer or peripheral part, 
coloured light blue, appearing as an envelope of greater or less 
thickness for the uncoloured part (fig. 46, a and d). These 
bodies are always attached to the chromatin network, and 
sometimes there appears about them a membrane derived 
from, and continuous with, the fibrils with which they are 
connected. This is very distinctly seen in the safranin pre- 
parations, the membrane in this case exhibiting a combination 
