C. Dobell and M. W. Jepps 
329 
stands on its four corners in the middle of the concavity, with its film 
downwards. The corners are then firmly cemented to the block with 
drops of paraffin wax. The preparation is then placed on the stage of the 
microscope and the film examined under a low power. When a cyst of 
E. histolytica has been found, it is carefully placed under the oil immer¬ 
sion in the centre of the field, and measured with the ocular micrometer. 
When the gelatine has set, the fixative is introduced with a fine pipette 
under the coverglass, and allowed to fill completely the space between it 
and the block. When this operation is properly performed, the cyst, 
which is observed continuously during the process, will be found to 
remain in position and not to be dislodged by the entry of the fixative. 
Its measurements after fixation may then be again recorded. 
In carrying out the above process, the following points should be kept 
in mind, since the success of the experiment depends upon them. The 
gelatine solution used must be of such concentration that it will liquefy 
at a very low temperature (so as not to injure the cysts) but set at the 
ordinary temperature of the laboratory. The concentration of salts in it 
must also be such that living cysts are unaffected by it. We found a 
5 % solution of gelatine in physiological saline solution (0-75 % sodium 
chloride in distilled water) to fulfil these requirements perfectly. Cysts 
in this medium appear perfectly normal under the microscope, and 
remain, in both the sol and the gel, unstainable with eosin (Kuenen and 
Swellengrebel’s test of the vitality of cysts). We tried a jelly deeply 
coloured with eosin in the expectation that the cysts would be more 
sharply defined, and therefore easier to measure, in this medium. We 
found, however, that the colourless jelly is preferable. The film must be 
made of a suitable thickness, which can only be determined by a few 
trials. It must not be so thick as to prevent exact measurement of the 
cyst after fixation of the jelly, nor so thin that the gelatine rapidly 
undergoes desiccation after it is set. The fixative must be introduced 
when the gelatine has completely set, but before it has had time to dry. 
The setting of the gelatine can easily be determined under the microscope 
by observing the complete cessation of Brownian movement in the 
smallest particles in the preparation. 
We made a number of observations by this method, and give below 
the results in tabular form (see Table III). The fixative used in all these 
experiments was that described above (sublimate-alcohol-acetic-acid). 
In the table the first measurement given for each cyst is the vertical 
diameter, the second the horizontal. Fixation of the cysts as seen under 
the microscope was almost instantaneous. As a rule the measurements 
