328 
Races of Entamoeba histolytica 
the outline of the cyst wall sharply, on account of the opacity of the 
stained cysts and the surrounding bodies in the preparation. 
More direct evidence was obtained by measurements of the thickness 
of the wall of individual cysts. The cyst wall of E. histolytica is extremely 
thin, and consequently difficult to measure. We were able, however, to 
obtain some satisfactory measurements by means of the camera lucida. 
The cysts were studied under a 2 mm. apochromat (N.A. = 1-40), with 
compensating ocular 18, and with critical illumination from an achro¬ 
matic aplanatic condenser (N.A. = T40), the measurements being read 
off directly from the projected image at a magnification of 2500 dia¬ 
meters. The thickness of the cyst wall was measured in living cysts, in 
cysts plasmolysed in 10 % sodium chloride solution, and in fixed, un¬ 
stained cysts suspended in 70 % alcohol. The thickness of the cyst wall 
was found to be remarkably uniform. In no case, after making between 
30 and 40 measurements, was it found to be less than 0-5/x or greater 
than 0-6/Lt. The mean value for 20 such measurements was 0-506p.. It 
should be added that the cysts used for these measurements were of 
approximately the same size as those from case, E. 42, but were obtained 
from two different patients. 
These figures are in complete agreement therefore with the supposition 
made above, namely, that the decrease in size observable in cysts 
mounted in balsam is due chiefly to the invisibility of the cyst walls in 
this medium. If the thickness of the cyst wall is 0-5-0-6p, we should 
expect the diameters of cysts in saline or iodine—in which the cyst walls 
are visible—to be on an average greater by twice this amount (lp-T2p) 
than the diameter of those in balsam—in which the cyst walls are not 
visible. And, as we have already stated, the mean diameter of cysts in 
balsam was actually found to be approximately Ip less than that of 
cysts measured in saline or iodine solution. 
It is clear that a direct proof of the correctness of this interpretation 
could be obtained if individual cysts could be studied microscopically 
both when alive, and during all the processes of fixation, staining, and 
mounting. As noted above, we succeeded, after some failures, in devising 
a simple method of doing so. Our procedure is as follows. A small 
sample of faeces containing cysts of E. histolytica is mixed on a slide 
with a suitable quantity of gelatine jelly previously liquefied by warming. 
Before the gelatine has time to set, a small drop of this emulsion is spread 
in the middle of a coverglass (7/8" x 7/8"), and the latter placed inverted 
in the concavity of a hollowground glass block (6 cm. x 6 cm., 1-25 cnr. 
thick, with a deep concavity 5 cm. in diameter). The coverglass thus 
