Mast, Light Reactions in Lower Organisms. 147 



following explanation. "The orientation of the colony may be 

 accounted for, if we suppose that the eye-spots are most sensitive 

 to light striking them at a certain angle such as is indicated in the 

 diagram by the lines a — b and e — /. If rays of light enter the 

 colony in the direction of the lines a — h and c — d somewhat 

 obliquely to the long axis, A — P, the flagella of the cells repre- 

 sented on the upper side of the diagram would beat more vigorously 

 and accelerate the motion of that side of the organism. The 

 opposite cell being struck by rays in the direction c — d would be 

 less stimulated, and, as the flagella would beat less strongly than 

 those on the other side of the colony, the organism would swing 

 about until its long axis isbrought parallel with the rays when, being 

 equally stimulated on both sides, it would move in a straight 

 course towards the light. We do not have to suppose that each 

 cell makes a special effort to orient itself at a particular angle to 

 the rays, but that it is so organized that the eff^ective beat of the 

 flagella is most accelerated by light striking the cell at a certain 

 angle. If the cells were most stimulated by light falling upon 

 them at such an angle as would result if the rays diverged from a 

 spot in front of the colony and in line with its long axis the con- 

 ditions for orientation would be fulfilled. Since the eye-spots in 

 all the cells face the anterior end of the colony this supposition 

 appears very probable. The foregoing explanation of the orienta- 

 tion of Volvox may or may not be the true one, but it enables us to 

 see a significance in the peculiar arrangement of the eye-spots in 

 this form and is consistent with the results of the experiments we 

 have described." Is it also consistent with the results of the 

 experiments described in the preceding pages .^ 



In the first place the eye-spots, upon the arrangement of which 

 Holmes places considerable importance in his theory, are not so 

 situated that they all face the anterior end; quite the contrary, 

 they face the posterior end of the colony, as pointed out on p. 107; 

 and in the accompanying diagram by Holmes they should be on the 

 side of the zooids nearest the end P, instead of on that nearest the 

 end A. They do, however, probably function as light recipient 

 organs, as already stated (p. 108). Let us then assume that the 

 zooids are influenced by the direction of the rays as Holmes sug- 

 gests, even if the eye-spots do face the posterior end of the colony, 

 and see if the theory fits our experimental results. 



I. It was clearly demonstrated (p. 139) that if specimens of 



