324 S. J. HOLMES. 



requirements of the theory. Orientation, however, occurs, ap- 

 parently equally well, in those individuals which contain so many 

 daughter colonies that a large proportion of the light is inter- 

 cepted in passing through the organism ; this explanation must, 

 therefore, be dismissed even if it be otherwise valid. How then 

 is the orientation of Volvox brought about? This problem is 

 one rather more difficult to solve than it might seem. There is 

 a suggestive similarity between the phototaxis of Volvox and 

 the reactions of this organism to the electric current. Carlgren 

 has found that Volvox orients itself very precisely to the con- 

 stant current and swims in very nearly a straight path to the 

 kathode. After a prolonged action of the current-a-more or less 

 pronounced tendency to go towards the anode asserts itself, but 

 the latter form of electrotaxis is much less precise and charac- 

 teristic. It seems not altogether improbable that light in passing 

 through a nearly transparent organism like Volvox exercises a 

 directive effect upon its movements in a similar way, whatever it 

 may be, to that produced by the current of electricity. The 

 direction of the rays may be the important factor in orientation 

 irrespective of differences of intensity of light upon different parts 

 of the organism as has been maintained by Sachs for the photo- 

 tropic movements of plants. I am not ready to adopt the theory of 

 Sachs, but I feel that it is a view that is not entirely out of court. 

 There is one feature of the structure of Volvox which may be 

 of some significance in relation to the problem of orientation. As 

 Overton has pointed out, the red eye spots of the cells are so 

 placed that they all face the anterior end of the colony. When 

 this end is directed toward the light the spots are in a position to 

 receive more than the usual stimulus. Orientation of the colony 

 would be produced if each cell were to react in such a manner 

 as to cause the eye spot to face the light. We have little direct 

 evidence, aside from a single experiment of Englemann on Eu- 

 glcna, that the so-called eye spots of the Flagellata are photo- 

 recipient organs, but there are certain facts regarding the occur- 

 rence of these spots as well as their arrangement in Volvox which 

 render it probable that such is their function. We may conceive 

 that each cell of the colony tends to orient itself at a different 

 angle to the rays of light, the cells of the anterior end where the 

 largest eye spots occur placing themselves with their long axes 



