TRYPANOSOMES IN TSETSE-FLIES AND OTHER DIPTHRA. 197 
the retractation of the flagellum is complete. Meanwhile the 
cyst-wall, which made its appearance as a cap round the 
posterior end of the body, is secreted more and more towards 
the anterior end, and when the flagellum has completely dis- 
appeared, the cyst is formed round the anterior termination 
of the body, thus producing the characteristic pear-shaped 
cysts which are very abundant in the preparation (fig. 198). 
This appears from Prowazek’s description to be the final 
stage of the cyst in the case of Herpetomonas; not so in 
T. grayi. The cysts, at first pear-shaped, with unlike ends, 
become more oval, with ends alike or scarcely distinguishable 
(figs. 199, 200), and finally they become more or less circular 
in outline, with the wall of even thickness all round (fig. 202). 
This stage is apparently the ripe cyst, and the last stage that 
can be observed in the body of the fly. 
While the process of encystation is going on, noteworthy 
changes are taking place in the nuclei. The trophonucleus 
becomes resolved into chromidia. Usually one large, irregular 
mass of chromatin can be seen, together with a variable 
number of irregularly scattered chromatin grains (fig. 197). 
At the same time the kinetonucleus diminishes in size, 
apparently also as the result of fragmentation. Amongst the 
chromatic grains scattered in the body, some can be dis- 
tinguished by their reddish colour, like that of the nucleus, 
others by their purple colour, like that of the kinetonucleus 
(fig. 195); the latter are fewer in number. It is important 
to note that the cyst substance also stains a reddish tint, 
similar to that shown by the granules of the nucleus, and it is 
possible that the disruption of the nucleus is in relation to the 
secretion of the cyst. 
In the ripe cysts the kinetonucleus seems to disappear com- 
pletely. In some of them it can still be made out plainly 
(fig. 202). In others it cannot be identified with certainty, 
but amongst the chromatic grains some can be seen which 
exhibit a more purple tinge than the others, and which 
represent probably the kinetonucleus broken up. The signi- 
ficance of these changes could only be made out by studying 
