On Pleodorina illinoisensis. 277 



distinct types of cells in the colony (PI. XXXVI., Fig. 1), 

 the vegetative (v. c.) and the gonidial (g. c.) cells. The 

 presence of these two types of cells at once places this new 

 species in the genus Pleodorina rather than in Eudorina — 

 which it otherwise closely resembles. 



The vegetative cells (v. c.) are four in number and consti- 

 tute the anterior polar circle, being always directed forward 

 in locomotion, as in the other species of the genus. Their 

 number remains the same in the smaller colonies of sixteen 

 cells and in the larger ones of sixty-four. The diameter of 

 these cells ranges from 9.5 to 16.8 /■<, twelve cells averaging 

 12.25 //. The size of these cells varies even in the matured 

 colonies, measurements at this stage ranging from 9.(5 to 

 15.6 //. At birth the cells of the young colonies vary in 

 diameter from 3.5 to 5 )i in different parents. In the 

 daughter colonies while still in the maternal matrix, no dis- 

 tinction in size between the vegetative and gonidial cells can 

 be detected, nor can this distinction be made in the younger 

 free-swimming colonies, it being thus impossible at this 

 stage to distinguish the young P. illinoisensis from the simi- 

 lar stages of Eudorina elegans with which they were asso- 

 ciated. AVhen the young colonies have attained dimensions 

 of 46x38 //, the vegetative cells measure 4 /< and the gonidia 

 4.8 //. A like similarity between the two kinds of cells in the 

 young colonies exists, according to Shaw ('94), in 1'. cali- 

 fornica. 



In structure the vegetative cells (PI. XXXVI., Fig. 2) are 

 in most particulars similar to the gonidia, described below. 

 They sometimes appear to be a trifle lighter green in color— 

 a difference which may be due to their smaller size. The 

 principal differences lie in the smaller number of pyrenoids 

 and the larger size, both absolute and relative, of the stigma 

 or eye-spot. 



As to the fate of the vegetative cells, the evidence at hand 

 is insufficient and conflicting. In three colonies in which 

 the daughters were moving about in the maternal matrix, 

 some having already escaped, the vegetative cells showed 

 very evident signs of degeneration, the contents being 



