On Pleodorina illinoisensis. 287 



rence of pleomorphism in the family Volvocinecs, Klein ('89 

 and '90) citing no less than twenty-four "combinations" in 

 the case of Volvox aureus. It may then be that the form here 

 described as Pleodorina illinoisensis is only a " Pleodorina 

 stage" of Eudorina. 



The abrupt disappearance of this supposed new species 

 from the plankton prevented the carrying out of breeding 

 experiments designed to test its validity, and it seems that 

 the matter must remain undecided for the present. In the 

 absence of satisfactory proof that the form here described is 

 but a phase of the life cycle of Eudorina it has seemed best 

 to the writer to make the above suggestion and to take the only 

 course open in publication, namely, the description of the form 

 as a new species, inviting the criticism of subsequent investiga- 

 tion. The dilemma here presented is by no means an isolated 

 one in plankton work, nor is it new to the family Volvocinece : 

 witness the long confusion which existed over the two species 

 of Volvox, aureus and globator, which has been at last cleared 

 up by the excellent work of Klein ('89, '89a, '90) and Overton 

 ('89). Another instance is often presented when Pandorina 

 and Eudorina both occur in the same collections and the 

 plankton statistician must decide to which genus each speci- 

 men observed must be referred. Typical specimens of each 

 can be found, but all individuals do not conform to the type, 

 or they may present conditions in which the conformation is 

 obscured by some phase of the life cycle. 



The asexual reproduction of Pleodorina illinoisensis (PI. 

 XXXVII.) resembles that of other species of the genus in that 

 it is accomplished by the repeated division of the gonidial 

 cells, resulting in the formation of daughter colonies in the 

 maternal matrix. These escape later from the parent organ- 

 ism, and by growth attain the adult condition with the differ- 

 entiation of the four vegetative cells. Five successive cell di- 

 visions, pervading all the cells of the parent organism except the 

 vegetative cells, are necessary for the completion of the process, 

 and result in the 2-, 4-, 8-, 16-, and 32-cell stages of the form- 

 ing colonies. The firsttwo of these divisions result in the forma- 

 tion of a quadrangular plate of cells — a form which is retained 



