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



witness tlie long confusion which existed over the two species 

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

 up by the excellent work of Klein (1889, 1889 a, and 1890) 

 and Overton (1889). 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 specimen 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. VI.) 

 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 

 divisions, 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 forming colonies. The first two of these divi- 

 sions result in the formation of a quadrangular plate of cells 

 — a form which is retained through the two succeeding 

 divisions, which produce the 8- and 16-cell stages. The 

 cupping of this plate, which results in the formation of an 

 ellipsoidal colony, is apparent as early as the 4-cell stage 

 (PI. VI. fig. 9) and continues through the later stages 

 (figs. 11, 13), so that by the time the 16-cell plate is formed 

 it has almost the curvature of a saucer. With the formation 

 of thirty-two cells the closure of the cup proceeds and is soon 

 completed. The orifice of the cup is directed outward in all 

 cases, and thus the ends of the cells of the daughter colony 

 which are formed from the outer end of the maternal gonidial 

 cell come to lie in the inner side of the cup, and are the inner 

 ends of the cells of the daughter colony. In the matured 

 colonies the young usually lie with their long axes parallel 

 to the surface of the parent. I have not, however, been able 

 to identify the point of closure of this cup with this region 

 or positively with any other. 



The sequence and position of cleavage planes which produce 

 the quadrangular plate of the 16-cell stage is, in the main, 

 similar to that described by Goroschankin (1875) and Braun 

 (1875) for Eudorina and Volvox. Beyond this stage there is 

 some doubt as to the agreement. A full discussion of the 

 subject is beyond the scope of the present paper, for which the 



