40 University of California Puhlications in Zoology [Vol. 16 



The method by which the new cytostome is formed is by no means 

 clear. The posterior peristome is often much thickened at the meta- 

 phase (pi. 6, fig. 26) and may be even heavier than the anterior 

 peristome. We find no evidence of the division of either of these 

 chromatic lines. They are apparently formed de novo on the new 

 side of the daughter somatella. The four instances (pi. 7. figs. 31-34) 

 of plasmotomy following mitosis (pi. 6, fig. 30) are not wholly in 

 agreement in their evidence on this point. The peristomal margins on 

 the upper left and lower right side of the figure are heavier (older) 

 in both zooids in figure 31, a.s though twisting at the cytoplasmic 

 bridge had placed the parental peristome on opposite sides. In figure 

 32 (dorsal view) the outer peristomes, right on the lower and left on 

 the upper, are heavier and presumably ancestral, while the inner are 

 faint or missing and are apparently in the process of formation. In 

 figure 33 both are heavy on the lower zooid, and absent or lightly 

 developed in the upper, as though one daughter had taken the whole 

 peristomal equipment and the other wa.s forming it anew. In figure 

 34 the anterior peristome alone is present and in both daughters is 

 rather light. 



The behavior of the parabasals during the process of mitosis and 

 binary fission is still somewhat problematical. It is doubtless com- 

 plicated by multiple fission and the possibility of moribund or patho- 

 logical conditions. We have one clear ease of its duplication, presum- 

 ably by division in the metaphase, since the two pairs of parabasals 

 are of equal size and adjacent (pi. 6, fig. 28). In other comparable 

 .stages of the same or later phases the pair of parabasals cannot be 

 found (pi. 6, fig. 30), and in some instances .scattered granular 

 chromidial masses are found in the cytoplasm in the region where the 

 parabasals normally occur. It is also absent in encysted stages (pi. 7, 

 figs. 34-43). Its disappearance may therefore be incidental to the 

 process of encystment and be correlated with changing conditions of 

 metabolism. 



The process of nuclear mitosis is carried on wholly within the per- 

 sistent nuclear membrane as in the trichomonad flagellates. The suc- 

 cessive phases established for metazoan mitosis are approximately 

 recognizable in this protozoan nucleus. The prophase is introduced 

 by the intranuclear chromidial cloud (pi. 5, figs. 4, 6) whose subsequent 

 disappearance (pis. 5, 6, figs. 8-17) is accompanied by an increased 

 .stainability of the cytoplasm (figs. 14, 15) with slight trace of extra- 

 nuclear chromidia, and by increased stainability of the peristome. 



