28(5 Illinois State Laboratory of Natural History. 



We also find that the structural differentiation shown in the 

 decadence of its posterior stigmata obtains in varying degrees 



in the other spherical and ellipsoidal genera of the family — 

 least in Pandorina, most in Volvox. The gemis Pleodorina 

 agrees with Volvox in having a structural polarity based upon 

 tlic division of the colony into vegetative and gonidial regions, 

 but the differentiation is simpler. Of the two species of Pleo- 

 dorina, the one here described exhibits the simplest possible 

 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 (' ( .»4), 

 who founded the genus, that it was intermediate between 

 Eudorina and Volvox but nearer the former. Judging merely 

 from the asexual stage, P. illinouensis 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 ;it least the vegetative cells may divide, one case of a 

 2-cell stage having been seen in the hundreds, if not thou- 

 sands, 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 occur- 



