12 S. O. MAST 



expect in positive specimens a shock-reaction in the zooids on the 

 side of the colony farthest from the source of Hght, owing to the 

 shading of this substance, but none on the opposite side where 

 this substance becomes exposed to the Hght. This is precisely 

 what is observed in the process of orientation. In colonies in 

 the negative state, on the other hand, we should expect just the 

 reverse and this is also in accord with our observations. More- 

 over, as the colony turns toward the light the change of intensity 

 on the hyaline portion of the eye-spot becomes gradually less 

 and when it has turned enough so that it directly faces the light, 

 that is, when it is oriented, there is no longer any change, and 

 consequently on the basis of our assumption no more shock-re- 

 actions and no further turning would be expected. This is again 

 in accord with our observations. If, then, the photo-sensitive 

 substance is confined to the hyaline portion of the eye-spot, or 

 largely so, and if the orienting stimulus is dependent upon the 

 time-rate of change of light-intensity on this substance, we can 

 account for the observed reactions in the process of orientation, 

 in Gonium, and this is in full accord with our explanation of ori- 

 entation in Stentor, Euglena, and a number of other organisms 

 (Mast, '11, pp. 80-135). Moreover, if our assumptions are cor- 

 rect, it is no longer necessary to hold, as in earlier publications 

 (Mast '11, p. 133), that the function of the eye-spot is not the 

 same in all organisms. But how on the basis of these assump- 

 tions is it possible to explain the fact that after the colony is ori- 

 ented it continues in a fairly direct course toward the light? 



This question has already been answered, in part, in the state- 

 ment that after the colonies are oriented changes of light inten- 

 sity on the hyaline substance in the eye-spots cease, and conse- 

 quently, if our explanation of orientation holds, no further turn- 

 ing would be expected, and the colonies should therefore remain 

 oriented, unless some factors other than the light in which they 

 are oriented cause them to turn. And if this should occur the 

 orienting stimulus would, in accord with our explanation, again 

 act, and result in reorientation. This explanation is based upon 

 the well known principle that organisms not subjected to lateral 

 stimulation tend to move in direct paths. To account for con- 



