284 Hazen: Lire HISTORY OF SPHAERELLA LACUSTRIS 
from a temperature of 18° even to the point where the surface of 
the water became coated with ice the zooids all the time sought 
the lighter side of the glass dish. Some of the variations in ex- 
periments with zooids may be due to currents in the water, for 
Sachs (’76) was able to produce many of the phenomena exhibited 
by zoóspores by means of minute oil globules. 
Microzooids are usually more positively attracted to light 
than megazooids, so that in cultures where both are present, as 
Sachs (’76) has noted, the microzooids are sometimes found only 
on the more illuminated margin while most of the megazooids ac- 
cumulate on the other side. I have found that, when taken from 
the light of a north window and placed in bright sunlight at a south 
window, megazooids will at first be neutral or even shun the light, 
but later will recover from the shock and swim toward the more 
intense light ; red zooids recover most quickly and young green 
. ones more quickly than older ones of the same color. The micro- 
zooids, on the other hand, from the very first swim toward the 
increased light, and when the slide is turned around they instantly 
turn and swim to the newly illuminated side. If, while they are 
moving from one side to the other, the sunlight is cut off they will 
go to all parts of the drop, but when the sunlight is again ad- 
mitted they will immediately swim toward it. 
Zooids transferred from the light of a north window to bright 
sunlight will not live for more than an hour, and the microzooids 
usually not so long as that, though they thrive when cultivated in 
sunlight from the first or when the change is gradual. 
CONDITIONS AFFECTING VITALITY 
The difficulty of obtaining a long cycle of development in drop 
cultures, owing probably to insufficient aération, has prevented the 
determination of the number of generations that may be produced 
ina cycle. There is every reason to believe, however, as Braun 
and Cohn supposed, that a considerable number of successive mo- 
tile generations may be formed in a colony. The colony may re- 
main in the motile state for a few days or even for three or four 
weeks. The length of this period depends in some measure, at 
least, upon the food supply. My best results have been obtained 
in cultures containing decaying vegetable matter; somewhat less 
