EYE-SPOTS 631 



stimulation. It is also worth noting that in Volvox, according to 

 Klein, brightly-coloured eye-spots only occur in the cells surrounding 

 the anterior pole of the colony. 



The only experiments so far carried out regarding the functions of 

 eye-spots, are those of T. W. Engelmann. This observer states that if 

 an individual of Euglena viridis, swimming in bright light, is partially 

 shaded, cessation and subsequent reversal of the movement only takes 

 place when the colourless anterior end which contains the eye-spot 

 enters a dark region. " In Euglrna, therefore, the light-perceiving 

 faculty is strictly confined to the colourless anterior end of the cell." In 

 this case, however, the response may begin before the eye-spot itself is 

 darkened. The actual perceptive power must, therefore, be vested in 

 the colourless cytoplasm, or in some portion thereof, perhaps, as Wager 

 suggests, in the bi-convex thickened region of the flagellum, which lies 

 just in front of the eye-spot. Similarly, in the case of swarm-spores, it 

 is most probably the colourless cytoplasm in the immediate vicinity of 

 the eye-spot which is responsible for the actual perception of light. 

 The eye-spot itself is, in fact, in all probability nothing more than an 

 auxiliary apparatus. The most obvious suggestion regarding the func- 

 tions of the eye-spot is that it acts as a light-screen, which prevents the 

 sensitive protoplasm from being illuminated equally on all sides, and 

 which thus helps to indicate the direction of the incident light ; if this 

 view is correct, the eye-spot might be compared to the pigmented 

 layers of animal eyes. 



