STRUCTURE AND LIFE-HISTORY OF COPROMONAS SUBTILIS. 99 
vidual. In the first case (1), the animal gradually becomes 
more rounded in shape, and decreases considerably im size. 
The flagellum is finally drawn in, and a very delicate cystic 
membrane is formed. A considerable time elapses during 
these changes, and I have never been fortunate enough to be 
able to observe the changes which occur in the living animal 
inside the cyst. These can be very clearly made out in stained 
specimens, however, and are described below. 
The reason why the contents of the cyst are so difficult to 
distinguish is probably to be sought in the manner in which 
the reduction in size is brought about. I think this is prob- 
ably effected by the protoplasm giving up part of its water. 
By doing so the cytoplasm would, for a time at any rate, 
become relatively denser, and so render the nucleus less 
distinct. 
It is not easy to see the cyst when first formed. It is only 
visible as a pale border to the rounded-off cell, and is not 
easily distinguished from a mere optical effect which makes a 
halo appear round a small, brightly illuminated object. The 
cysts appear more distinctly after some hours’ time, and seem 
to be composed of a soft, gelatinous substance. I have found 
that the most raprd way of demonstrating their presence at 
this and earlier stages is to proceed as follows: the hanging- 
drop containing the cysts is carefully spread out, after re- 
moving the coverslip, and allowed to dry rapidly. It is then 
fixed for ten minutes in absolute alcohol, and stained for ten 
minutes in Giemsa, then rinsed in water, blotted with a 
cigarette paper, and mounted in cedar oil. By this method 
the cysts are stained pink, the protoplasm inside dark blue, 
with the nuclei sometimes visible as dark blue, purple, or red 
bodies. 
The gelatinous cysts, in the normal course of events, become 
hard and slightly yellowish in colour. They are’ not un- 
commonly encrusted more or less completely with the minute 
organic particles which abound in the feces cultures. ‘There 
is a good deal of variation in the shape of the cysts, some of 
the commonest forms encountered being shown in figs. 17, 18, 
