KOFOID AND SWEZY: UNARMORED DINOFLAGELLATA 65 



With the formation of a cyst wall the body shrinks somewhat in size, leaving 

 a space between itself and the containing cyst (fig. I, 2). Division of the 

 nucleus takes place by a process of mitosis, and is usually repeated twice before 

 the body divides, so that four nuclei are present at the first division of the 

 cytoplasm, which resiilts in the formation of four daughter cells. The nuclei 

 of these daughter cells divides again (fig. I, 3), producing eight nuclei or even 

 sixteen with a fourth division. With the division of the cytoplasm eight or 

 sixteen daughter cells are produced. These elongate and become encysted in 

 slender crescent-shaped, thick-walled cysts which are completely filled b}^ the 

 body when first formed (fig. I, 4). 



The mother or primary cyst ruptures soon after the completion of the for- 

 mation of the daughter or secondary cysts which float out freely into the water. 

 No evidence has been forthcoming to show that these secondary cysts are capable 

 of independent motion when the rupture of the primary cyst wall releases them. 

 On being set free the next stage in their development takes place. The cyto- 

 plasm of the encysted cell soon begins to shrink away from the wall (fig. I, 3), 

 concentrating at the center of the body around the nucleus, which divides 

 (fig. I, 6) once, twice or three times (fig. I, 7). The small bodies thus produced 

 develop into the t.ypieal Gymnodinium with a full complement of furrows and 

 flagella. These escape with the rupture of the cyst wall and form the motile 

 individuals of the cycle. They may become encysted (fig. I, 9) and divide 

 during the encysted state (fig. I, 10) or while free-moving. Simple binary 

 fission may be repeated many times, probably before the next stage is entered 

 upon, which results in the production of the phase shown in figure I. 



Division of the contents of the secondary cysts into eight individuals is not 

 an invariable rule, apparently, since cysts are sometimes found containing two, 

 three, four, or six fully formed Gtjmnodininm, individuals. This is probaldy 

 the result of the delay or omission of the second division or the division of one 

 or more secondarily formed nuclei. In some of the larger species (pi. 2, fig. 14) 

 a single individual may be formed within the cyst. The result of the process 

 outlined in figure I is normally the production of 64 or 128 individuals from 

 a single cell. The length of time necessary for the completion of this process 

 has not thus far been determined. Dogiel (1906) has estunated the length of 

 time for the development of the sixteen crescent- shaped cysts (fig. I, 4) from 

 an individual in which the first division has taken place as six hours. This 

 part of the life cycle is probably much longer than the part resulting in the 

 production of the large spherical form from the small Gymnodinmm, since it 

 is relatively common, while the other stage has thus far- eluded observation. 



The crescent-shaped cysts have been found in several species of Gymnodi- 

 Hiitm and also in Goui/aitJax. One of these belongs to the larger species of 

 Gijmuoduiiuni as figured l)y Pouchet (1885a, pi. 2, fig. 4), with a single large 

 individual within the cyst." The form shown by Schiitt (1895, pi. 25, fig. 808, 

 reproduced in our text fig. Q, 1 ) as Gymnodinium Jiiiinla is obviously not related 



