162 
Weston.—-The Development of 
pure water, zoospore emergence takes place ; in the presence of nutriment, 
germ tubes are formed ; and when, after hypha-formation has begun, the 
available nutriment is suddenly exhausted or removed, dwarf sporangia are 
produced. Frequently, also, substances which increase the osmotic pressure, 
but are without nutrient value, seem to exert the same influence as nutri¬ 
ment in this connexion. 
Zoospore emission occurs as follows: The sporangiospore content 
emerges rapidly through a softened papilla of escape as an amorphous 
mass (Fig. 15), which assumes the characteristic structure of the motile 
zoospore. Near the already apparent nucleus (Fig. 16) a contractile 
vacuole develops, while the surface becomes grooved by a depression 
(Fig. 17) from which, close to the nucleus, arise two delicate cilia that lash 
slowly back and forth. This lashing increases in violence until the zoospore 
finally frees itself with a sudden jerk, and swims actively away (Fig. 18). 
In the case of sporangiospores retained in situ , the emerging zoospores are 
imprisoned by the enveloping sporangium membrane or by surrounding 
sporangiospores. As a result, sporangiospores, zoospores, and cystospores 
may at times be observed within the same sporangium wall (Fig. 29). 
From retained sporangiospores in contact with the sporangium wall, 
zoospores often emerge through the wall as in the genus Dictyuchus 
(Fig- 3 °)- 
The zoospores are of the laterally biciliate type found in most Sapro- 
legniaceae. Under high magnification the inadequacy of the commonly 
used terms ‘ bean or ‘ kidney shaped becomes obvious. Precise description 
is difficult, but the zoospores may be characterized roughly as subovoid and 
as bearing a longitudinal furrow. Since the customary figures of this type 
of zoospore are somewhat misleading, an attempt has been made to present 
the solid form in the accompanying diagrams (Text-fig. 2). In size the 
zoospores average about 12*5/x in length and 9 ijl in width. Their loco¬ 
motion, involving progression and rotation on the long axis, is in an irregular 
spiral course through the water. The more tapering end is directed forward, 
and by the tractile action of the shorter anterior cilium the zoospore 
is drawn rapidly along, while the longer cilium is dragged behind as 
a more or less passive rudder. Although rotation is generally to the right, 
it may be reversed or may cease, while the zoospore moves irregularly 
forward. These changes in motion seem to be dependent upon the activity 
of the posterior cilium, although the precise method of lashing is not 
distinguishable. To chemical and other stimuli the zoospores respond much 
as did the Saprolegnia zoospores investigated by Pfefifer ( 23 ) and Stange 
( 27 ). Duration of the motile period is not fixed. A tendency to motility, 
together with a sensitiveness to those influences of the environment that 
result in encystment, is inherent in each zoospore. In pure water a 
zoospore may swim about for. 30 or 45 minutes, but at any minute, if 
