212 C. CLIFFORD DOBELL. 
formation have been negative. Neither change of tempera- 
ture nor nutrition of the host appears to have the slightest 
influence. When I had almost despaired of ever finding the 
cysts I suddenly came upon them—in apparently quite 
ordinary frogs. It is curious—though perhaps a mere co- 
incidence—that the cysts were all found in the months of 
November, December and January, before, and in part con- 
temporary with, the period of cyst-formation in Opalina. 
When the cysts are present they are usually found in fairly 
large numbers, for many animals encyst at the same time. 
Before encysting the animal undergoes considerable changes 
as regards its nuclear structure. Instead of the chromatin 
remaining distributed in the form of fine granules through- 
out, it begins to concentrate in the centre and in the nuclear 
membrane. ‘The result is the formation of a nucleus with a 
sharp chromatic outline and a very distinct karyosome (fig. 
13). A delicate thread is usually to be made out running in 
a longitudinal direction and uniting the karyosome with the 
membrane above and below (cf. fig. 13). 
When the animal has reached this stage it begins to round 
itself off and decrease in size, preparatory to secreting a cyst 
wall. This process takes a very long time, so that it is almost 
impossible to follow it out in one and the sameanimal. How- 
ever, | have seen every stage in different animals so many 
times that there can be no doubt about what occurs. The first 
thing that happens is that the axostyle begins to disappear, 
gradually dwindling away from behind forwards. As the 
caudal process ceases to exist the animal is able to round 
itself off. It does so, coming slowly to rest. After a time 
the movements of the flagella get slower and slower, and 
finally cease. Then the flagella disappear. They seem to 
dissolve, but it is difficult to see what becomes of them. It 
is just possible that they are drawn into the body, as in 
Copromonas in division. ‘lhe blepharoplast remains behind, 
lying upon the nucleus (fig. 14). A diminution in size takes 
place, so that the organism shrinks to an oval mass of proto- 
plasm. In this stage it forms the cyst membrane (fig. 14). 
