150 Dr. C. A. Kotoid on Pleodoviiia illinoisensis. 



precise statement as to locomotion in this genus. As ob- 

 served by nic, it closely resembles th;it described above for 

 Pleodorina illinoisensis ; viz., rotation around the lon:^ axis 

 of the colony, the same pole constantly leading in progression. 

 The direction of rotation is frequently reversed, though it was 

 predominantly from right over to left in the cases observed. 

 A functional polarity thus exists in this genus. 



In Vohox^ according to Klein (1890), there is a polar differ- 

 entiation as regards the stigmata that is even more marked 

 than it is in the genera previously mentioned. He finds that 

 the cells of the pole directed forward in locomotion each 

 possess a stigma which is especially large and intensely 

 coloured ; that the colour fades out and the stigmata become 

 smaller and paler as the equator is approached ; and that 

 beyond this they are usually represented merely by a colour- 

 less oil- drop, which in some cases may even disappear. The 

 posterior hemisphere is also marked by the development 

 there of the gonidia, as was first shown by Cohn (1856), and 

 occasionally ellipsoidal colonies are found whose long axis 

 connects the anterior and ])ostcrior poles. Locomotion in 

 Volvo.T is accomplished, as elsewhere in the family, by the 

 rotation of the colony about its principal axis. Wills (1880) 

 observed the predominance of the rotation to the right and 

 its occasional brief reversal. Klein (1889) states that this 

 |)reference is found in V. globator, but that it is not shown by 

 V. aureus. In this latter species the changes are frequent and 

 are often separated by a brief pause. Backward motion is 

 rarely seen and lasts but a short time. In tlie case of Volvox 

 the axis of rotation is slightly oblique, the centre of the 

 colony remaining in the line of progress, but the axis of 

 rotation being inclined from above the line at the anterior 

 polo to below it at the posterior one. 



We thus find that Pleodorina illinoisensis, which exhibits 

 both a structural and physiological polarity, shares with 

 most, if not all, of the genera of the family to which it 

 belongs the physiological differentiation which is expressed 

 in locomotion, and also, in observed cases, exemplifies the 

 extreme form of a predominance of rotation in one direction. 

 We also find that the structural differentiation shown in the 

 decadence of its posterior stigma obtains in varying degrees 

 in the other spherical and ellipsoidal genera of the family — 

 least in Pandvrina, most in Volvox. The genus Pleodorina 

 agrees with Volvox in having a structural polarity based upon 

 the division of the colony into vegetative and gonidial regions, 

 but the differentiation is simjder. Of tlie two species of Pleo- 

 dorina, the one here described exhibits the simplest possible 



