Dr. C. A. Kofoid on Pleodorina illinoisensis. 153 



following brief description must suffice. The first cleavage 

 plane (I) divides the gonidial cell into two hemispheres along 

 the axis of the cell, and the daughter nuclei, with the sur- 

 rounding protoplasm, are placed close together in the centre 

 of the opposing faces of the new cells (PI. VI. fig. 7). 

 The second plane (II) is at right angles (figs. 8, 9) to the first 

 and also passes through the regions representing the axis 

 of the ancestral cell. In this instance also the nuclei are 

 gathered near the centre of the young colony, which exhibits 

 to an appreciable extent the curving indicative of the later 

 formation of the cup. The 8-cell stage results from the 

 divisions of each of the quadrants of the 4-cell stage by a 

 plane (III) which is parallel to one of the previous planes 

 and perpendicular to the other, meeting the latter at a point 

 about midway between the centre and the circumference. By 

 a subsequent adjustment of the cells the four more centrally 

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

 are filled by the other four (PI. VI. figs. 10, 11). The 16- 

 cell stage is formed by four additional planes, each of which 

 divides one of the cross-cells and its corner neighbour. 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 circumference 

 toward the centre. 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 over- 

 hangs a row of three cells, of three grades of elevation (PI. VI. 

 figs. 12, 13). The succeeding division and the completion of 

 the cup (PL VI. 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 recognizable 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 im- 

 probable that these structures also must undergo some division 

 during the process of cell-multiplication. During the pro- 

 cesses of division the nuclei continue to occupy a position 

 near the inner ends of the cells (in the new colony), and it is 

 only alter the divisions have been completed that they come 

 to occupy their usual positions at the centre 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 



