0)1 IHeodor'uia illiitoiscnsis. '287 



ivnce of pleomorpliisni in the family ViAvoc'nuu/', Klein ('89 

 and '90) citing- no less than twentj'-four " comhinations " in 

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

 described as Pleodoriiut tUinoisensis is only a " Vlcodorhia 

 stage" of Kt((Ioriii((. 



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 Kudorlna 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 subsecpient investiga- 

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

 one in plankton work, nor is it new to the family Volvoclncir -. 

 witness the long confusion which existed over the two species 

 of Volvor, unrcuH and (jlohntor, 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 Pandoriiui 

 and Eudorhia both occur in the same collections and the 

 plankton statistician must decide to which genus each speci- 

 men observed must l)e 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 Pleodorlna lU'nwiHensia (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 diti'er- 

 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 '1-, 4-,-<S-, 16-, and 32-cell stages of tl'e form- 

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

 tion of a (piadrangular plate of cells — a form which is retained 



