Dr. C. A. Kofoid on Pleodorina illinoisensis. 151 



differentiation of the colony consistent with the symmetry 

 of the organism ; viz., the differentiation of the anterior 

 polar circle of four cells as vegetative members of the colony. 

 Of the. two species of the genus it thus stands nearer Eudo- 

 rina, while its sister species, P. californica, approaches more 

 closely to Volvox both in the number of cells and in the 

 extent of the differentiation. 



The discovery of this additional species of the genus Pleo- 

 dorina thus supports the opinion expressed by Shaw (1894), 

 who founded the genus, that it was intermediate between 

 Eudorina and Volvox but nearer the former. Judging merely 

 from the asexual stage, P. illinoisensis affords additional evi- 

 dence of the close relationship of Pleodorina and Eudorina. 



Throughout the preparation of this paper the writer has 

 had constantly in mind the possibility that the form here 

 described is merely a stage in the life cycle of Eudorina. A 

 number of facts lend support to this hypothesis : (1) the 

 occurrence of Pleodorina illinoisensis with Eudorina elegans ; 

 (2) their marked similarity, aside from the four vegetative 

 cells, in structure and measurements; (3) the impossibility 

 of separating the youngest free-swimming colonies of the 

 two forms; (4) a considerable variation in the size of the 

 vegetative cells in Pleodorina^ grading toward the condition 

 in Eudorina; (5) some evidence that in certain cases at 

 least the vegetative cells may divide, one case of a 2-cell 

 stage having been seen in the hundreds, if not thousands, 

 of specimens examined, and one instance noted in which 

 a maternal colony containing thirty-two daughter colonies 

 had at one pole four colonies which were slightly smaller 

 than the remaining twenty-eight ; and (6) the occurrence 

 of pleomorphism in the family Volvocinete, Klein (1889 and 

 1890) 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 Volvocinese : 



