299 Hazen: Lire History OF SPHAERELLA LACUSTRIS 
rock during the present winter. Sometimes I have found large, 
scattered cells, at other times numerous clusters of small cells 
clearly indicating recent division, but never any zooids, even 
though at times there was sufficient water for movement. The 
reason here again appeared to be that the more abundant water 
supply was accompanied by so low a temperature that the motile 
state was not acquired. Material collected from the rock at such 
times and placed in a slightly warmer atmosphere produced abun- 
dant zooids (Figs. 23, 45-48). 
Rostafinski (’75), indeed, reports that when he placed vessels 
containing many zoóspores outside a window at a temperature be- 
tween 6? and 2? multiplication and production of zoospores con- 
tinued in normal fashion; when during the night the temperature 
fell and-a mass of purple ice was formed, he found after slow thaw- 
ing, a large number of zoóspores which showed active vibratory 
movements. I have repeatedly attempted to confirm these results 
but I have never been able to discover zooids produced at a tem- 
perature lower than 12?—15? C. I have always found that if a 
vessel containing zooids is slowly frozen the zooids may still con- 
tinue to live under a crust of ice where the temperature of the 
water is about 1? C., but when actually imprisoned in ice they 
invariably die. Cohn (’50) also found that motilé cells were killed 
by freezing. It is quite possible that Rostafinski thawed his cul- 
tures so slowly that new zooids were formed. 
I have never found any evidence of any other type of vege- 
tative propagation than the endogenous division already described. 
Cohn's figure 15 which has been copied by Bennett and Murray 
and others as the palmella-condition of Protococcus may be inter- 
preted as a case of endogenous division in which the daughter-cells 
have been pressed together so as to be bounded partially by flat 
surfaces, while the mother-cell-wall has disappeared. 
It is to be concluded then, that vegetative division plays an im- 
portant part in the multiplication of Sphaerella lacustris, but that 
the process does not differ from that by which zooids are produced 
except that the motile condition is not acquired because of insuffi- 
cient water, aération, or temperature. 
