C. Dobell and M. W. Jepps 
333 
In all the experiments hitherto described, we have confined our 
attention to cysts of large size (with diameters usually of about 12-14/u.). 
Our conclusions, therefore, are directly applicable to cysts of these 
dimensions only. The difficulty of manipulating very small cysts, and 
the much greater difficulty of discounting errors in the measurement of 
objects of very small dimensions, deterred us from attempting to extend 
our observations to cysts of smaller size. It seemed to us almost certain, 
however, that closely comparable results would have been obtained for 
these. We have assumed, therefore, when dealing with the smaller- 
sized cysts of E. histolytica, that the effects of mounting in balsam would 
be the same, and the apparent reduction in size proportionate. 
The foregoing experiments supply, we think, conclusive solutions of 
the problems which we set ourselves. We may therefore now summarize 
our results up to this point. 
1. Cysts of E. histolytica which have been suitably fixed and stained 
and mounted in balsam, appear to possess a smaller mean diameter than 
those examined alive in saline solution, or after treatment with iodine. 
(There appears, however, to be no appreciable difference between the 
size of living cysts suspended in saline and those killed and suspended 
in Weigert’s iodine solution.) 
2. When properly fixed the cysts do not undergo any appreciable 
shrinkage in the fix;ative or in the usual reagents employed in staining 
and dehydration. The apparent loss of size takes place in the process of 
clearing and mounting; and is chiefly due to the disappearance of the 
cyst wall when the cyst is placed in a medium of high refractive index. 
Shrinkage of the protoplasmic contents from the cyst wall may also, in 
some cases, account for a still further diminution in the apparent 
diameter, 
3. The average thickness of the cyst wall of E. histolytica was found 
to be 0-5/x. In an ideal case, therefore, in which no protoplasmic shrink¬ 
age occurs, and the apparent diminution of the diameter of the mounted 
as compared with the living cyst is due only to the disappearance of the 
cyst wall in the mounting medium, the difference in diameter should be 
approximately 1 fi. Comparative measurements of living cysts and of those 
mounted in balsam with due precautions to prevent shrinkage and dis¬ 
count its effects, actually gave a difference of this magnitude in the mean 
diameters of the cysts. Shrinkage of the protoplasm may, however, lead 
to a considerable further reduction in size. (In one case directly observed 
under the microscope there was a reduction in the mean diameter of 2-6p,.) 
