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RESEARCHES ON FUNGI 



to Mast,^ the eye-spot in the zooid of a Gonium or a Volvox (Fig. 54) 

 consists of three parts : a pigment-cup, a lens which is situated at 

 the opening of the cup, and a photosensitive substance which is 

 located within the cup. Mast ^ regards these eye-spots as " primitive 

 eyes " and, in connexion with Volvox, explains their action as 

 follows. 3 Volvox swims in the direction of its axis with its anterior 

 end forwards.* Each of the zooids of which it is composed has an 



Fig. 54. — Camera-lucida drawing of an optical section through the longitudinal axis 

 of a colony of Volvox : l-a, longitudinal axis ; G, gelatinous layer surrounding 

 the colony ; H, hyaline portion of cell ; Z, zooid ; n, nucleus ; /, flagella ; 

 c, chloroplast ; e, eye ; p, pignaent-cup ; /, lens ; mm, scale. Note that the eyes 

 on the two sides of the longitudinal axis face in opposite directions. Copied by 

 the author from Fig. 6 of Mast's Reactions to Light in Volvox ivith Special Reference 

 to the Process of Orientation. In the original Figure the chloroplasts are coloured 

 green, and the pigment-cups red. Reduced to three-quarters. 



eye-spot and two cilia. The eye-spots are largest at the anterior 

 end of the colony (about 3 jj. in diameter) and decrease in size from 

 that end backwards so that, at the posterior end, they are so poorly 

 differentiated as to be scarcely visible. In the eye-spots at the 

 anterior end of the colony the lens is directed away from the axis 

 of the colony and slightly forwards (Fig. 54). The cilia of all the 



1 S. 0. Mast, " Reactions to Light in Volvox with Special Reference to the 

 Process of Orientation," Zeitschrift fiir vergleichende Physiologie, Bd. IV, Heft V, 

 1926, pp. 648-649, 657. 



2 Ibid., pp. 652, 657. ^ Ibid., pp. 643-657. 



* This statement applies only to photopositive colonies. There are also photo- 

 negative colonies which swim away from the light. 



