AO8 DORIS L. MACKINNON. 
suggested, the effects of high and low temperature on the 
nucleo-cytoplasmic proportions in the Actinospherium cyst. 
Briefly stated, the normal course of encystation, as observed 
by Hertwig, is as follows:—Hncysting Actinospheria with- 
draw their pseudopodia, adhere to the substratum, and take 
on an opaque, greyish appearance. ‘This is the stage of the 
mother-cyst; during it a silica coat begins to form, and 
about 95 per cent. of the nuclei present in the unencysted 
state disappear. The mother-cyst divides up into a varying 
number of primary cysts, each containing one nucleus: that 
is to say, the number of the primary cysts formed by a given 
Actinospherium depends on the number of nuclei remaining 
over from the mother-cyst reduction. Hach primary cyst — 
gives rise by division to two secondary cysts. From the 
nucleus of each of these, two polar bodies are given off. ‘The 
secondary cysts then fuse, two and two, each pair of nuclei 
forming together the single nucleus of a conjugation-cyst. 
From this conjugation-cyst the free-swimming Actino-sphe- 
rium is finally liberated. Hncystation can be artificially 
induced by starving Actinospheeria that have previously been 
feeding well. 
In April I prepared starvation-cultures from fresh Actino- 
spheerium material fetched from a pond at Possenhofen, near 
Munich. I chose for my purpose 90 or 100 individuals of 
approximately similar size, and kept half of them in closely 
covered watch-glasses of clean culture-water at a temperature 
of 10° C., the other half at 25° C. I allowed them no food, 
and merely from day to day removed impurities, and put in 
fresh water. When encystation had occurred, as happened 
usually within three or four days, I killed them off at the 
primary-cyst stage, and substituted fresh cultures. (In the 
beginning I kept a third line of cultures going at an inter- 
mediate temperature of 17°C. Soon, however, as the weather 
erew warmer, it was impossible to keep the room where the 
cultures stood at this low temperature, and I gave up the 
attempt.) me; 
At first, though encystation took place rapidly and nor- 
