212 Hazen: Lire HISTORY oF SPHAERELLA LACUSTRIS 
University of Vermont, and to Professor L. M. Underwood, of 
Columbia University, for their encouragement and aid in this 
study ; and to the several botanists and zoólogists who have 
furnished information regarding the distribution of Sphaerella 
and especially those who have sent specimens for comparison. 
GENERAL ACCOUNT 
Sphaerella lacustris is usually found in urns or shallow pools 
formed in rock hollows which are either periodically filled with rain 
or supplied by water oozing from overhanging ledges. In these 
pools a blood.red crust composed of an infinite multitude of 
minute spherical cells covers every loose fragment of rock and the 
sides of the basin. The bottom is usually less densely coated, 
partly because the motile forms in seeking air and light are left at 
the edges of the water as it evaporates, and partly because the 
plants resting on the dirt accumulating at the bottom are more 
easily washed out by storms than those adhering to the solid rock. 
If a portion of the red crust after being scraped off and dried 
for a short time is placed in a dish of water over night, it will be 
found in the morning that many of the cells are in process of divi- 
sion. During this process the cell mass, whose inner contents 
are entirely obscured by the blood-red pigment, increases in 
size and elongates and the outer layer of the thick cellulose 
cell-wall ruptures and allows the inner layer to be pushed out in a 
bladder-like expansion (Fig. 3). This gives space for growth and 
also facilitates the complete rupture of the membrane. Division 
proceeds. until four, eight, or sixteen daughter-cells are produced. 
These daughter cells increase somewhat in size by absorption of 
fluid and usually I have found that they form very delicate celi- 
walls about themselves before leaving the mother-cell-wall (Fig. 
5). Each develops a pair of cilia which may sometimes be seen 
fifteen or twenty minutes before the time of escape. 
In some way a rent is made in the attenuated part of the 
mother-membrane, or less frequently it becomes softened and the 
daughter-cell bursting out rapidly swims off, ciliated end foremost. - 
These zooids which are often angular and irregular in shape at 
first, soon become ovoid in: consequence of the pressure from 
within and the tension of the cilia. The forward movement is ac- 
