On Pleodorina illinoisensis . 289 



a subsequent adjustment of the cells the four more centrally 

 placed ones come to form a sort of a Greek cross whose angles 

 are filled by the other four (PL XXXVII., Fig. 10, 11). The 

 16-cell stage is formed by four additional planes, each of 

 which divides one of the cross-cells and its corner neighbor. 

 The location of these planes may be described in the same 

 terms as the last excepting that they meet the radial planes, I 

 and II, at about one fourth the distance from the circum- 

 ference toward the center. The cupping of the plate soon 

 advances to such an extent that it consists of a square of four 

 centrally placed cells, upon each of the four sides of which 

 there overhangs a row of three cells, of three grades of eleva- 

 tion (PI. XXXVII., Fig. 12, 13). The succeeding division and 

 the completion of the cup (PL XXXVII., Fig. 14) result in the 

 young colony's assuming the ancestral form. Throughout 

 these divisions the number of pyrenoids in the daughter cells 

 grows steadily less. But one can be found in each cell in the 

 32-cell stage, while in the 16-cell stage two are readily recog- 

 nizable in each cell. In the earlier stages and before division 

 the number often varies, and the pyrenoids are frequently so 

 crowded that enumeration is difficult if not impossible. It 

 seems not improbable that these structures also must undergo 

 some division during the process of cell multiplication. Dur- 

 ing the processes of division the nuclei continue to occupy 

 a position near the inner ends of the cells (in the new colony), 

 and it is only after the divisions have been completed that 

 they come to occupy their usual positions at the center of the 

 cells — perhaps as a result of the growth of the chromatophore. 



No stage of sexual reproduction has been positively identi- 

 fied for this species. 



A peculiar condition of the colonies of this species, also 

 occurring in Pandorina and Eudorina, deserves passing- 

 notice. It occurred with considerable frequency in all three 

 genera and resulted in each case in the destruction of the 

 entire colony affected. The early stages of the disease, if it 

 be such, are indicated by the homogeneous condition of the 

 cells and the fading out of the color, together with a flattening 

 of the cell contents into a disk- or lozenge-shaped mass (PL 



