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



difFerciitiation of the colony coiisistciit with the symmetry 

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

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

 Of the two sj)ccios of the genus it thus stands nearer EaJo- 

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

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

 extent of the differentiatioii. 



'J'he discovery of this additional species of the genus P/eo- 

 don'ua thus supports the opinion e.xpressed by Shaw (1894), 

 who founded the genus, that it was intermediate between 

 KuJorina and VoJvo.r but nearer the former. .In l^^ing merely 

 from the a.-^oxual stage, P. illi'iioisensis affords additional evi- 

 dence ol the close relationshI|) of Pleodorina and Eudorina. 



Throughout the prcj^aration 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 

 oecui rence 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 pk-'omorphism in the family Volvocine;e, Klein (1889 and 

 1890) citing no less than twenty-four "combinations " in the 

 case of Vol cox 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 |>lanklon 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 [)roof that the form here described is 

 but a phase ot the life cycle of Eudorina it has seemed beat 

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

 course open in j)ublieation, namely, the description of the form 

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

 tion. The dilemma iiere jneseuted is by no means an isolated 

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



