16 S. O. MAST 



6. SUMMARY 



1) The eye-spots in both Gonium and Eudorina consist of an 

 opaque cup-shaped part and a hyahne lens-shaped part. The 

 latter is partially surrounded by the former and it is probably 

 relatively very sensitive to changes in light-intensity. These 

 changes are probably largely due to shadows produced by the 

 opaque part. 



2) Orientation in Gonium is direct. The colonies never turn 

 in the wrong direction, as often occurs in Euglena, Stentor, and 

 many other forms. The turning which results in orientation is 

 due to an increase in the activity of the flagella on the zooids 

 farthest from the sources of light. In these zooids the hyaline 

 part of the eye7spot is shaded by the opaque part at the time the 

 activity of the flagella increases. 



3) If the light-intensity of the field is suddenly decreased, the 

 rate of locomotion, in positive colonies, suddenly increases. But 

 if it is slowly decreased or if it is increased there is no response. 

 In negative colonies, however, just the reverse is true. They 

 respond to a sudden increase in rate of locomotion if the illu- 

 mination is decreased but not if it is increased. 



4) The increase in the activity of the zooids on one side of the 

 colony during the process of orientation is apparently of the same 

 nature as the increase in the activity of the whole colony when 

 the illumination is changed. This indicates that orientation 

 in these organisms is dependent upon the time-rate of change of 

 light energy on the photo-sensitive substance, probably the 

 hyaline portion of the eye-spots, and not upon the absolute 

 change or the continuous-action of light. 



