Br. C. A. Kofoid on Pleodorina illinoisensis. 143 



can be seen to penetrate the matrix of the parent very slowly. 

 Their ends are often blunted, or even knob-like, and their 

 lateral motion is very limited. The movement of the young 

 colonies through the matrix is a very slow and gradual one, 

 showing the gelatinous consistency of the substance in which 

 they are imprisoned. 



The striking feature in the structure of this species, as in 

 the case of P. calif or nica (Shaw, 1894), is the presence of 

 two distinct types of cells in the colony (PI. V. 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 other- 

 wise 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 yu, twelve cells averaging 

 12*25 fx,. The size of these cells varies even in the matured 

 colonies, measurements at this stage ranging from 9*6 to 

 15"6 fjb. At birth the cells of the young colonies vary in 

 diameter from 3*5 to 5 fi in different parents. In the daughter 

 colonies, while still in the maternal matrix, no distinction 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 similar stages of Eudorina 

 elegans with which they were associated. When the young 

 colonies have attained dimensions of 46 x 38 fi the vegetative 

 cells measure 4 fi and the gonidia 4'8 /a. A like similarity 

 between the two kinds of cells in the young colonies exists, 

 according to Shaw (1894), in P. calijornica. 



In structure the vegetative cells (PI. V. fig. 2) are in most 

 particulars similar to the gonidia, described below. They 

 sometimes appear to be a trifle lighter green in colour — 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 shrunken 

 and irregular. In the larger number of instances of this stage 



