58 University of California Publications in. Zoology [Vol. 20 



In the "resting" nucleus four parts may be distinguished (pi. 6, 

 fig. 11). The center is occupied by a linin reticulum closely filled by 

 a mass of chromatin granules. These are sometimes large and closely 

 ma-ssed together, or they may be small and show a definite linear 

 formation (pi. 6, fig. 14). In the predivision stages the latter 

 arrangement becomes the common one and results in the formation 

 of the chromosomes (pi. 6, figs. 14^18). Near the outer border of 

 this central mass and often slightly imbedded within it, is a small 

 vesicle surrounded by a very thin membrane. This contains a single 

 small, coiled or twisted i-od of chromatin (fig. B, heterochr. ; pi. 6, 

 figs. 11, 14, 16), surrounded by a clear area which does not stain. 

 The position of this vesicle varies somewhat in different individuals. 

 It is most frequently found near the end of the longer axis (pi. 6, 

 figs. 11, 14, 19), but may lie at the side of the nucleus near the end 

 of its shorter axis (pi. 6, fig. 16). Its size also varies somewhat as 

 well as that of the chromatin rod contained within it which we have 

 designated heterochromosome for convenience. In plate 6, figure 11, 

 the vesicle is relatively small and nearly filled by the chromatin rod. 

 In figure 16 of the same plate, both the vesicle and the chromatin 

 rod are relatively large. 



Outside of these two nuclear regions is a zone of large, clear 

 alveoli (pi. 6, fig. 11). The walls of the alveoli seem, in some cases, 

 to be continuous with the linin reticulum of the central area, but 

 otherwise the two regions are distinct. The alveoli are rounded out- 

 wardly and pressed close together on their inner faces. They are 

 filled with a clear fluid which does not stain. Outside of this alveolar 

 zone and separating it from the outer membrane is a granular area, 

 the rather coarse granules of which stain lightly with iron haema- 

 tosylin. This granular portion may have a width equal to half that 

 of the alveolar zone (pi. 6, fig. 11), or it may narrow down to a thin 

 line (pi. 6, fig. 14). In many individuals both the granular and 

 alveolar regions may be almost indistinguishable, the central chroma- 

 tin mass nearly filling the entire nuclear spaces. 



In its structure the nucleus of this flagellate recalls that of Gyro- 

 dinium coralUnum (Kofoid and Swezy, 1919c?, pi. 10, fig. 117) ; the 

 latter, however, has been observed only in the living condition. In 

 this it presents a similar alveolar zone surrounding a central mass 

 of granules. The presence of a small vesicle with its chromatin rod, 

 or heterochromosome, has not been observed in this species. 



