C. Dobell and M. W. Jepps 
327 
merely to the circumstance that the cyst wall is clearly visible in the 
former medium, but almost always invisible in the latter. The diameter 
of a cyst measured in saline therefore includes twice the thickness of the 
cyst wall, whereas that of a cyst mounted in balsam is really the diameter 
of the stained protoplasmic contents only. This supposition is supported 
by the measurements made of cysts in euparal. As is well known, the 
refractive index of euparal is lower than that of Canada balsam dissolved 
in xylol; and accordingly the walls of cysts mounted in the former 
medium are frequently visible, and would therefore be often included in 
measurements of their diameters. We should therefore expect that the 
mean size of cysts mounted in euparal would be intermediate between 
the sizes of those in saline and those in balsam. This we found to be the 
case, the mean size of cysts in euparal being 12-2p,. If this explanation 
is correct we should expect further that the cysts actually measured in 
balsam would, if remounted in a medium of lower refractive index, be 
found to have a greater mean size. To test this we remounted in water 
the coverglasses which had been originally mounted in balsam, and from 
which the above measurements had been made. Five hundred cysts were 
again measured in them, and the mean size was found to be 12-3p. as 
against 12-04/x for the same cysts in balsam. This increase in size, though 
appreciable, was not so great as we had expected: but we considered 
that the measurements of the cysts in water were not altogether trust¬ 
worthy. It was often extremely difficult, if not impossible, to distinguish 
Parasitology x 22 
