The Cyst of Amoeba proteus. 207 
following description of the living cyst is based upon optical sections, through 
the débris, after as much as possible of this has been removed. 
The walls of the cyst are three in number, if the outer envelope of débris 
be included in the term “wall.” There is little doubt but that its constituents 
are cemented together by the exudation from the amoeba while encysting, 
and it is thus a part-product of the creature for self-defence and not an 
accident. Whether this first formed wall would occur if the amoeba could 
be induced to encyst in the absence of débris, I have not been able to prove 
satisfactorily up to the present. The two remaining, or rather the true walls, 
of the cyst are membranous, the outer being the older. In the earlier stages 
these two walls are spherical and well filled by the amoeba-body, but as 
maturity is reached there is usually a contraction of the protoplasm within 
the younger membrane. 
There is no reason for assuming that the space between the older 
membrane and the outer envelope is occupied by anything more than a 
watery fluid—that is to say—there is no thick hyaline wall, as described by 
Scheel in the case of the cyst studied by him, and termed the “glassy zone.” 
When an encysted amoeba was turned out of its envelope of débris, it 
- became clear that the older membrane was quite free from this wall by the 
way in which the spherical amoeba rolled out, and moreover, if the two 
membranous walls were complete, a second pricking combined with delicate 
pressure would set the amoeba with its youngest wall quite free from the 
outer, and in no case were there visible the jagged edges of anything other 
than a fine membrane. 
With regard to the internal contents, as has been already mentioned, the 
granular protoplasm and the form of the nucleus can sometimes be seen, but 
when the early stage is passed nothing can be distinguished. A ripe cyst has 
more than once been fixed and sectioned which showed no features to mark 
it off from the empty cysts by which it was surrounded, so transparent had 
its contents become. 
Tn size the cysts agree with Scheel’s measurements, averaging in life from 
70 » to 140 p» in diameter, but after fixation the contraction leaves the 
sectioned cysts from 60 » to 110 » in diameter. 
METHOD. 
The material when received from the pond was carefully examined 
for cysts. If any were found they were isolated from the stock, which was 
then left undisturbed for several days to favour the encystment of any 
unencysted amoebz, many of which were always present. Of the isolated 
