215 
Species of Olpidiopsis , (Cornu) Fischer. 
of these closely related genera may reveal that the above-described relation 
of the cilia is common, if not the general rule. 
As has been described for other species (Butler, 4) there occurs in the 
life of the zoospores two distinct motile periods. After their escape from 
the sporangium they swim about actively for a few minutes, two to five, in 
the vicinity, and then gradually come to rest upon the slide or cover-glass, 
as the case may be. There is now seen a waving rhythmic movement of the 
cilia, accompanied by a gentle rocking of the zoospore. These movements 
become slower and slower, the cilia gradually shorten, and the body of the 
spore assumes a spherical form. Finally, at the end of seven to ten 
minutes the zoospore has become quiet, and the cilia, when visible at all, 
appear as short thickened knobs or projections about the length of the 
radius of the spore. 
The period of rest may last only a few or as many as fifteen or twenty 
minutes, during which time one, sometimes two, vacuoles are seen to 
appear and pulsate regularly — in one observed case, at intervals of six to 
eight seconds — until the second period of motion begins. The reverse 
condition now takes place. The short cilia begin to move and lengthen, 
the body of the zoospore resumes the rocking motion, and in a few minutes 
darts away. There is no change in the position of the cilia, and the fact 
that, in many cases at least, they do not lose their identity during the 
resting period makes such an assumption seem improbable. Butler (4) 
failed to observe cilia on zoospores of P seudolpidiiivi Aphanomyces during 
the first period of motion when this was followed by a second. The fact that 
such were seen attached laterally on zoospores killed in the sporangium at 
the time of discharge led him to state (p. 133) : ‘ From this it would 
appear probable that there is no alteration in the position of the cilia during 
the two periods of motion, but it is possible that cases occur in which the 
first period is suppressed, and the sporangium figured may have been such 
a case.’ He further states : ‘ As in P. Saprolegniae , the occurrence of two 
periods of motion is not, I think, a constant phenomenon.’ 
Butler suggests that such a condition may be compared to diplanetism 
in the Saprolegniaceae and Pythium. Inasmuch as diplanetism seems 
to prepare the zoospore for a more or less continued period of activity, and 
that in the cases just described the second period may persist for some time, 
it seems that, so far as it concerns the Saprolegniaceae, such a comparison 
is, at least, worthy of comparison. 
C. Infection of the Host. 
Inasmuch as the observations of the writer on the penetration of the 
host by the zoospores of the parasites differ immaterially from similar 
studies made by previous investigators, it. seems unnecessary to give more 
than the most important facts as they apply to the species under considera- 
