132 
Proceedings of the Royal Society 
say that they are enclosed in cell-membranes, however well the lens 
be focused and the light adjusted. No doubt, the well-defined border 
would suggest such a structure, but it cannot be seen. 
Because no membrane is apparent, it does not follow that one may 
not exist. If a glass tube be filled with water, and held between 
the light and the eye of the observer, the column of water will 
appear thicker than it really is. This depends upon the fact that 
the refractive index of the glass is greater than that of the water. 
Now, it will be easily seen that the glass tube might be of such a 
thickness that the column of water will appear as thick as the outer 
border of the tube itself ; in fact, the tube will be no longer seen. 
In the case of the liver-cell, a like explanation may account for the 
apparent absence of a structure, the presence of which the sharp 
contour, remaining so even after distortion of the cell, would seem 
to suggest. These considerations led me to an investigation of the 
subject, and I have been able to demonstrate the existence of an 
investing membrane by a very simple procedure. With the point 
of a scalpel a scraping is taken from the unhardened liver of an ox ; 
this is mixed on a glass slide with a small drop of magenta fluid, 
and it is then covered. A slip of blotting paper is folded and 
refolded until a thick square pad is formed, which is placed over 
the cover-glass. With the handle of a needle or a pencil pressure 
is applied through the pad to the preparation, or it may be 
hammered, for there is little fear of breaking the cover-glass, the 
pressure being diffused through the paper. The object may be now 
examined with a power of 400 diameters. If the pressure has been 
too great all the cells will have been destroyed, and the whole field 
will be covered by a magenta-stained debris, in which disengaged 
nuclei are seen, in this case the preparation is of little use, and a 
fresh trial must be made. If successful, unbroken cells will be seen 
at the border of the cover-glass, a granular mass in the centre, and 
midway between the centre and border many half -broken ones, in 
which latter the most satisfactory evidence of the existence of the 
cell-membrane is to be sought. If one of these be examined, a 
granular mass will be seen projecting from the rent, or in close 
proximity to it. This is part of the contents of the cell which has 
been squeezed through the aperture. Generally, the cell still con- 
tains its nucleus, surrounded by the greater mass of the protoplasm. 
