THE INTESTINAL PROTOZOA OF FROGS AND TOADS, 218 
At first this is soft, but later it becomes harder and thicker. 
The axostyle gradually goes, and during the time of its dis- 
appearance a little darkly-staining, triangular area is gene- 
rally visible between its remains and the nucleus (fig. 14), 
The significance of this is not apparent, 
In the end the axostyle completely vanishes. The nucleus 
becomes drawn out in its long axis to such an extent that it 
often comes to stretch almost from one end of the cyst to the 
other (fig. 15), The karyosome also shares in the process, 
becoming drawn out into a long strand, which remains 
united to the membrane at either end, Above the nucleus, 
and in contact with it, the karyosome can generally be seen as 
a minute diplosomic structure (fig. 15). This stage is the 
last, and the cysts must now be regarded as permanent struc- 
tures, which probably serve for the dissemination of the 
parasite. Although I have had cysts under observation for 
weeks at a time they haye never undergone any further 
change. ‘This is not difficult to determine, because although 
very small their structure can be made out quite clearly— 
with proper illumination, etc.—in the living state, 
The cysts vary in size from ca. 4u-7u X ca. 4u-6y, but 
average dimensions are ca. 65 X ca. Su. 
The reduction in size in course of encystment is probably 
brought about by loss of water. It seems likely that before 
the reduction begins an actual diminution in the amount of 
solids in the composition of the protoplasm takes place, On . 
several occasions I found that the large animals which were 
about to encyst were extraordinarily hard to fix. Instead of 
fixing in the ordinary way they collapsed, leaving only a few 
shreds of protoplasm and nucleus behind. The smaller 
animals—those in later stages of encystment—were fixed 
quite well however. 
I have many times endeavoured to cause the animals to 
leave their cysts again, by treating them with the digestive 
juices of the frog. But allattempts have failed—a fact which 
I attribute to the abnormal condition of the laboratory frog, 
more especially in winter, when the experiments were made. 
