ETE-SPOT AKD FI/AGELLITM IN ETJGLENA VIEIDIS. 479' 



way to the exterior, passes quite close to it. Moreover it possesses 

 an enlargement on one of its basal supports which is placed im- 

 mediately in front of, and in close contact with, the inner concave 

 surface of the eye-spot. 



The presence of this enlargement of the flagellum in such a 

 position at once suggests the simple explanation that the light- 

 rays which are absorbed by the eye-spot cause a stimulation 

 of the enlargement in some way ; and this stimulation reacting 

 upon the flagellum causes its movements to become modified, and 

 so results in a change in the direction of the movement of the 

 cell. 



We have thus a combination of two structures, which may, 

 if my explanation be correct, be regarded as an extremely simple 

 form of eye, consisting of a specialized portion of the cell 

 {protoplasm ?) possessing great sensitiveness to light-rays of a 

 particular kind, and a pigment-spot, as a light-absorbing organ, 

 in close contact with it. 



"Whether it is the light absorbed by the pigment-spot which 

 actually effects the change in the movements of the flagellum ; 

 or whether the pigment-spot simply prevents these rays from 

 reaching one side of the enlargement, whilst the other side is 

 left freely exposed to them, thus producing a difference of 

 intensity on the two sides of the enlargement, it is at present 

 impossible to say. We know that protoplasm itself is sensitive 

 to light, and responds to it without the intervention of any such 

 structure as a pigment-spot. It may be therefore that the 

 arrangement just described in Euglena is one by which the 

 active rays are, under certain conditions, cut off on one side of 

 the enlargement, while the other is left exposed to them. This 

 would result in a definite unequal illumination of the sensitive 

 portion of the cell ; and consequently the organism would tend 

 to move into such a position that it would be more or less equally 

 illuminated all round, and would be brought ultimately into the 

 light-ray. 



I put forward this explanation tentatively, as the only one 

 which appears to me at present to afford an adequate inter- 

 pretation of the facts observed. It is evident, however, that 

 it is a question which demands a more extended and laborious 

 investigation than I have so far been able to undertake, before 

 any very definite conclusions can be formulated. 



