On Pleodorina illinoisensis. 281 



ceeds. This prominence of the stigmata in the anterior end, 

 together with the facts that this end is always directed fore- 

 most in locomotion and that the species showing this differ- 

 entiation are positively phototactic in the vegetative condition 

 when the differentiation is prominent, all point toward the 

 l)articipation of the stigmata in the function of light percep- 

 tion. An interesting phenomenon occurs at the time of the 

 division of the gonidia, for the stigma of the mother cell 

 persists and is passed on through the five successive cell 

 divisions to the outer end of one of the cells of the daughter 

 colony, situated in the margin of the cup which arises from 

 the plate of cells and closes to form the ellipsoidal daughter 

 colony. Inasmuch as this cup always closes from the inside 

 out, that is with the opening directed outward, it is evident 

 that the stigma must traverse the distance between the outer 

 end of the mother cell and its inner end, which corresponds 

 to the outer ends of the cells of the daughter colony. New 

 stigmata arise in the cells of the daughter colony, but being at 

 first very small are thus quickly distinguishable from the 

 persisting stigma. The ultimate fate of this persisting stigma 

 has not been traced. 



No contractile vacuole was observed in the living cells, and 

 careful search with a Zeiss ^.,-inch oil-immersion lens for 

 this structure in preserved and stained material has led to no 

 positive identification of a vacuole. The bleached stigma 

 and what seem to be the enlarged bases of the flagella are the 

 only areas discernible in the anterior end of the cell which at 

 all resemble a contractile vacuole. Shaw ('94) finds in picro- 

 nigrosin material a single vacuole in the anterior end of the 

 young cells of P. calif or n tea. 



The flagella (,/'.) are two in number for each cell, and unite 

 with the cell at the anterior end adjacent to the stigma. The 

 ttfo flagella have the same proportions, and in adult colonies 

 they measure 40 u in length. In the young colonies they 

 are relatively longer. They are visible on the young colonies 

 shortly after the cup closes, and persist upon the maternal 

 colony during the early divisions of the gonidia. 



The locomotion of the colonies of Pleodorina illinoisensis 



