Dr. 0. A. Kofoid on Pleodorina illinoisensis. 149 



In Stephanosphcera the polarity in structure is but slightly 

 marked in the colony, being indicated in some colonies by 

 the asymmetrical position of the cells, but there is a physio- 

 logical differentiation in that one pole of the colony leads in 

 locomotion. In this genus also, according to Colin (1852), 

 the rotation is in either of the two directions and is subject 

 to frequent change. No predominant direction was noted 

 by him. 



In Pandorina the only structural expression of polarity is 

 found in the greater development of the stigmata in the cells 

 in the anterior end of the colony. In other particulars the 

 poles are not differentiated. Braun (1851) maintains that in 

 this genus the rotation is constantly around the long axis of 

 the colony in the direction of the hands of a clock, when the 

 motion is toward the observer. Nageli (fide Biitschli, 1883- 

 1887, p. 858), on the other hand, observed rotation in both 

 directions. My own observations upon Pandorina morum 

 show beyond question that the direction of rotation is not 

 constant, as the following table demonstrates: — 



The table gives the direction and number of changes in 

 direction in rotation of ten colonies, each observed for one 

 minute. The plus and minus signs indicate the estimated 

 predominance in duration. According to the table the in- 

 stances of direction observed are approximately equal for the 

 two directions, though that from right over to left showed 

 the greater duration. The younger and smaller colonies 

 showed much the greatest activity and exhibited more 

 frequent changes in direction than the older colonies. In all 

 observed cases the same end continues to lead in locomotion, 

 physiological polarity being thus fully developed in this genus. 



In the case of Eudorina the structural polarity of the 

 vegetative colonies is no more marked than it is in Pandorina, 

 though according to Carter (1858) there is in the monoecious 

 sexual colony a differentiation, in that the four cells at one 

 pole divide to form spermatozoa, while the remaining twenty- 

 eight become egg-cells. It should be noted in this connection 

 that no such colonies were observed by Goroschankin (1875) 

 in the sexual generation. The literature at hand presents no 



