KOFOID AND SWEZY: UNAKMORED DINOFLAGELLATA 



71 



Some observations on collections made in the early morning lead to the 

 conclusion that fission takes place more frequently during the hours of night 

 or very early morning than at other periods of the day, indicating some relation 

 between nutrition and alternating darkness and light on the one hand and this 

 ■\-ital process on the other. Binary fission also seems to occur more frequently 

 during the life cycle than does the more complex process of multiple fission. 

 The former may occur in the unarmored forms in the motile state or after 

 encystment, the latter appearing to ])e the more common condition when fission 

 occiirs. 



Fig. N. Multiple fission in small Gyrodinium. 1. First ilivision with girdles forming. 2. Second division 

 with beginning of third in upper zooid. 3. Sixteen individuals with fully completed girdles, but still closely 

 attached to each other. X 400. 



Multiple fission has already been described as part of the life cycle of 

 Gjimnodinium Jutiuln (fig. I), and consists of successive mitoses of the body, 

 resulting in the ^jroduction of three to sixteen individuals within the same cyst. 

 Another type of multiple fission has occasionally been found in ovir material 

 which may represent another phase of the life cycle, or may be a specific modi- 

 fication of the process already described. This consists in the production of 

 few to many small individuals from one body within the ordinary oval, thin- 

 walled cyst (fig. N). Cysts of this t^-pe have been noted, which contained divid- 

 ing individuals of Gijmnodinium, Gyrodinium, and CocModinium. These ap- 

 parently differ from the secondary cysts of Gymnodinium lunula in that they 

 are more delicate, resembling in every respect the ordinary cyst enclosing a 

 single individual of any of these species (as in fig. Q). The secondary cysts of 

 G. lumda, on the other hand, are tough and highly resistant, mthstanding for 

 some time the action of preservatives of all kinds. It seems not improbable, 

 therefore, that a process of nudtiple fission may take place during the same 

 part of the life history as that in which binary fission occurs. 



Another type of multiple fission was noted in a single individual only. This 

 was an elongated encysted form, apparently belonging to the genus CocMo- 

 dinium, with the girdle still intact when first noted, though the sulcus had be- 

 come obliterated (fig. O, 1). The nucleus of this individual presented a late 



