478 ME. HAEOIiD WAGEE ON THE 



of their flagella or cilia, of responding quickly to changes in its 

 direction or intensity, an eye-spot is present. Those motile cells 

 which do not possess an eye-spot are either not sensitive to light 

 at all, or only to a slight extent. Even in the zoospores of 

 Chytridium, which according to Strasburger * are sensitive to 

 light, it is interesting to note that at the base of the cilium there 

 is a conspicuous orange-coloured oil-globule, which may act in 

 the same way as an eye-spot. 



Again, Engelmann has shown for Euglena and Strasburger for 

 swarmspores of Algae, that the rays of light which are most active 

 in their influence upon the movements of these organisms are 

 found in the region of the blue portion of the spectrum, and 

 these are just the rays which, as indicated by the colour of the 

 eye-spot, are absorbed bj it. We conclude from this, therefore, 

 that it is the light absorbed by the eye-spot, and not that trans- 

 mitted through it, which is concerned in these movements. 



Finally, it has been shown by Engelmann that it is the colourless 

 anterior end of Euglena t\\dtX \^ sensitive to light. "Hence, in 

 this case, a certain part of the body functions to a certain extent 

 as an eye "f. In this colourless anterior end of the cell, both the 

 eye-spot and the apparatus which directly causes the movement 

 of the cell — the flagellum — are placed. 



Having thus briefly stated the evidence we possess in favour 

 of the conclusion that the pigment-spot of Euglena is a definite 

 light-perceiving organ, we must now attempt some explanation 

 of the way in which the light acts. 



It is obvious that, whatever may be the action of light, the 

 movements of the cell, as well as any change in direction of its 

 movements, are dependent directly upon the flagellum. Without 

 it, as we have seen, the cell is only capable of a very slow con- 

 tractile movement, of its body from place to place. It is evident 

 therefore that those rays of light which are capable of exerting a 

 material influence upon the movements of the cell can only do 

 so by controlling or modifying in some way the mechanism by 

 means of which the flagellum is caused to move. Now we have 

 already seen that the flagellum and eye-spot are closely related 

 to each other. The flagellum arises near the eye-spot and, on its 



* Jena. Zeitschr. xii. 1878, p. 668. 

 1- Hertwig, ' The Cell,' p. 100. 



