Mast, Light Reactions in Lower Organisms. 107 



says they are about ^// apart and 25/x long. The eye-spot is situ- 

 ated on the surface of the projection which faces the posterior end 

 of the colony. It is found but a short distance from the free end, 

 between the points of attachment of the flagella. 



If the projections are short or absent and the zooids nearly 

 spherical the eye-spots are still located in the same relative position 

 as they are in zooids containing long projections, /. e.y they face 

 the posterior end of the colony. This becomes very evident in 

 viewing a colony from the anterior end. Under such conditions 

 it is clearly seen that the eye-spots are situated on the surface of 

 the zooids farthest from the middle of the anterior end, as repre- 

 sented in Fig. 3. 



Nearly all the investigators, who have worked on the structure 

 of Volvox, figure the eye-spot as situated on one side of the zooids 

 near the outer surface, but only one, Overton, describes and 

 figures it in such a way that its position with reference to the colony 

 as a whole is made clear. Overton ('89) in Taf. 4, Fig. 26, and 

 Taf. I, Fig. 3, clearly represents the eye-spot as being located near 

 the outer anterior surface of the zooids and says, p. 114: "Sehr 

 bemerkenswerth erscheint, dass, wie bei einstellung auf einen 

 Meridiankreis des Volvox Stockes sich ergibt, die Augen flecke 

 (wenigstens bei V. minor) bei alien Zellen derjenigen Seite anlie- 

 gen, die dem vorderen Pole am nahsten liegt." 



During the first few days in August, 1905, I examined 30 speci- 

 mens of V. globator and 50 of V. minor, with special reference to 

 the location of the eye-spots and found that in all but one of these, 

 they were unquestionably located on the outer posterior surface 

 of the zooids. Furthermore, I gave the problem of locating this 

 structure to three of my students in October, 1905. These stu- 

 dents had never seen Volvox before and knew nothing about any 

 work done on it. All of them concluded that the eye-spots face 

 the posterior end of the colony. When they took up the problem 

 they knew that these organisms are usually positive in their light 

 reactions. I had given them the term, eye-spot, and it was clearly 

 evident that they assumed that this structure functioned in direct- 

 ing the organisms toward the light, and consequently expected to 

 find it on the anterior surface of the zooids, for they were all much 

 surprised to find it on the opposite surface. It is, therefore, safe 

 to conclude that Overton's observation was wrong. 



The eye-spots in Volvox are brownish in color and lenticular in 



