1919] Rhodes: Binary Fission in Collodictyon triciliatum Carter 205 



night, the ice being one-eighth to one-quarter of an inch in thickness, 

 forming at a temperature of 28 F. Under the ice and throughout 

 the following day Collodictyon seemed even more numerous than ever 

 and many dividing forms were found. The following night, tempera- 

 ture 32 F, abundant mitotic stages of Collodictyon and Peridinium 

 were found. Many of the dinoflagellates escaped from their theca by 

 ecdysis and many division stages were observed. Greeley (1903) 

 noted the effect of reduction of temperature upon the artificial pro- 

 duction of spore formation and multiple fission in Monas. I obtained 

 nothing resembling resting spores or somatellas. At 4 C Greeley 

 (1903) found that Monas rounded up into a resting spore in about 

 six hours, and at 1 C multiple fission, resulting in a resting somatella, 

 was observed within five days. It is interesting to note in comparison 

 that binary fission was simply accelerated in both Collodictyon and 

 Peridinium at a temperature of 32 and 28 F. 



Collodictyon seemed most abundant on the surface, naturally tend- 

 ing to accumulate in the corners and around the edges of the aqua- 

 rium. But during the day and night at all temperatures above freez- 

 ing, I have found them present throughout the aquarium, from the 

 bottom to the top, under the protected area as well as the open end. 

 I placed slides and covers, suspended at various depths, as well as 

 covers suspended in cylinders to eliminate currents, and found 

 abundant organisms in all parts of the aquarium. In these experi- 

 ments, attempted primarily to determine the time and conditions of 

 division and probable multiple fission, I was led to conclude that 

 division was determined more by chemical than physical conditions, 

 that it was no more abundant at night than during the day, that 

 individuals undergoing binary fission remained at the surface for the 

 most part, but could be found at all depths, in all degrees of light and 

 temperature, though more abundant at 32 F. At no time have I 

 found a clear case of multiple fission. One instance (pi. 7, fig. 62) 

 of a somatella was observed, but on careful comparison of the stain- 

 ing reactions, I was led to conclude that this was a cyst of Amoeba 

 radiosa, vegetative stages of which were abundant in the aquarium. 

 The life cycle of Collodictyon, as far as traced, is thus simple, repro- 

 duction being by binary fission only. "When the organisms wholly 

 disappear from the cultures, I have been unable, by varying con- 

 ditions, to start the culture up again. It seems, therefore, that there 

 may be no cysts or resting stages for this flagellate, at least under 

 the conditions observed. The extreme variations in size (pi. 2, figs. 



