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



some of the posterior cells the colour is often very faint, giving 

 the stigma the appearance of a slightly tinged oil-globule. 

 It is of an elongated hemispherical shape when seen from the 

 side, and has a circular outline when seen from above. Its 

 upper end often projects slightly so as to elevate the cell- 

 membrane. The application of killing agents and alcohol 

 soon removes its colour, and even in formalin this fades out in 

 the course of a few days, leaving merely a colourless, highly 

 refractive structure. The larger stigmata have a diameter of 

 2"5 fJb and a depth of 2'8 /a, and are to be found in the cells 

 in the anterior part of the colony, especially in the four vege- 

 tative cells of the anterior polar circle. Posteriorly the stig- 

 mata are less prominent, and are often not to be found at all 

 as brightly coloured spots but merely as pale globules whose 



eal character, 



position alone affords a clue to their real 



This specialization of the stigmata in the anterior end of 

 the colony occurs also in Eudorina, Pandorina, and Volvox, 

 and Shaw (1894) states that in P. californica the stigmata, 

 which are present in the posterior part of the young colonies 

 (in gonidial cells), become less conspicuous and disappear as 

 the colony enlarges and the differentiation of the cells pro- 

 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 

 participation 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 



