307 
(fig. 34). Im some individuals the cyst-wall is secreted first at the poste- 
rior end and attains its full thickness there before itis completed around 
the body (figs. 36, 37); but in other cases it appears to be formed first 
at the sides (fig. 35). I have no indication as to whether the free flagel- 
lum is absorbed (retracted) or breaks off; the encysting parasites of 
figs. 34, 36, 37 may have been, of course, forms without any free portion 
of the flagellum. 
Fig. 34—37. 
Fig. C. (All figs. 2250. By an error these have been reduced rather more than 
was intended.) Encystment. 34—37, different stages in the process; 38—41, com- 
plete cysts. 
When I examined these infected female mosquitoes in the spring, 
before knowing of the occurrence of cysts, the subsequent natural destiny 
of the “Crithidiae” appeared very debatable. I kept two living prepa- 
rations of “rejuvenated”, active Flagellates, from two digestive tracts, 
one lot in water, the other in normal salt-solution. After 24 hours, only 
a small number of active individuals were still apparent; others were 
very languid, the flagellum moving feebly; a large number had died or 
disintegrated. After 48 hours, only a few solitary individuals could be 
found alive in the aqueous preparation, free in the water. But the in- 
teresting fact was noticed that a considerable number of parasites which 
had remained enclosed inside a portion of the digestive tract, namely 
the pyloric end of the stomach and the proximal region of the intestine 
(which had been left in the preparation), were still quite normal and 
active at this period; and several of these were still alive the following 
day, when there was no sign of living individuals in the water around. 
These observations certainly appear to indicate that the active parasites 
were not able to live for any length of time in water, outside the host; 
and this is only what was to be expected, bearing in mind that they were 
not encysted. There is another fact which makes it difficult to suppose 
that these individuals succeed in being taken up by larvae. In the 
great majority of cases, at any rate, there can be no larvae available to 
