REACTIONS TO LIGHT IN LARVAE OF ASCIDIANS 173 



that if the Hght from the mirror is cut off while tadpoles on a 

 slide are observed under a microscope, it can be seen that only 

 those become active in which the opening of the eye-cup faces 

 downward. This would indicate that the activating reactions to 

 reduction of light are due to a decrease in the illumination of the 

 nerve endings in the eye. 



I repeated these experiments, but was unable to obtain conclu- 

 sive results. By means of other methods, however, I obtained 

 results which confirm Grave's conclusion and demonstrate that 

 in addition to the activating reactions observed by Grave the 

 bending or directive reactions of the tail, which in the preceding 

 experiments were induced by changes in the illumination of the 

 field are also dependent upon impulses received through the eye. 

 In other words, it was found-that all of the reactions described in 

 the preceding pages can be induced by changes in the illumina- 

 tion of the retina without any change in the luminous intensity 

 of the field, or even with changes in the wrong direction, "showing 

 clearly that the eye functions in all of these reactions as a light 

 receptor. 



In these observations two methods were employed. One 

 consisted in studying the effect in a horizontal beam of light of 

 changes in the axial position of free-swimming specimens such 

 that different surfaces of the eye become exposed; the other con- 

 sisted in noting the reactions in attached specimens when the 

 source of light was changed in position so as to illuminate differ- 

 ent surfaces of the eye. In both it was possible to ascertain the 

 eff-ect of changes in the illumination of the retina. 



First method. If a number of specimens are put into a vase- 

 line enclosure under a cover-glass, some usually become attached 

 to the vaseline and others get into narrow spaces between the 

 cover and the slide so as to prevent locomotion. With such 

 specimens under the binocular in the dark-room, a limited por- 

 tion of any desired surface of the eye can readily be illuminated by 

 changing above and below the stage, the position of a small 

 electric bulb containing a concentrated filament. In this way, 

 numerous observations were made on the effect of increasing 

 and decreasing the illumination of the retina. The results 



