Mast, Light Reactions in Lozuer Organisms. 103 



to the colony as a whole, the other, the variation in form of the 

 vital portion of the individuals composing the colony. Since these 

 structures are of considerable importance in the study of light 

 reactions, I shall take up the structure of Volvox rather more in 

 detail than otherwise would be necessary. The following descrip- 

 tion is the result of a review of the literature on this subject, sup- 

 plemented by my own observations. 



Volvox varies in form from approximately spherical to ovoid. 

 The smallest free swimming colonies can scarcely be seen with the 

 naked eye, while the largest are nearly, if not quite, one millimeter 

 in diameter; Klein ('89, p. 143) gives 850;«, Hansgirg ('88, p. 

 loi) 800//, KiRSCHNER ('79) 700/i, and Focke ('47) iioo//. Some 

 of the investigators found Volvox globator to be larger than 

 Volvox minor, while others found the opposite to be true. 

 Klein gives 800// as the diameter of the largest colonies of V. 

 globator and 850^4 as that of the largest V. minor. Hansgirg 

 gives 800/1 as the diameter of the former and 460/i as that of 

 the latter. In my own collections I found V. globator in general 

 much larger than V. minor. I did not, however, make any 

 accurate measurements with reference to this point. 



The colonies of both species are composed of numerous individ- 

 uals, each of which consists of one cell. Klein ('88, p. 146) found 

 from 200 to 4400 individuals in various colonies of V. minor and 

 from 1500 to 22,000 in V. globator. The individuals consist of a 

 central portion, composed largely of protoplasm, and a thick hya- 

 line layer which surrounds the central portion. The central por- 

 tion will be referred to as the zooid in the future description. The 

 hyaline layers of contiguous cells usually appear continuous, one 

 with the other, but Williams ('53) demonstrated that they are 

 limited by cell walls. I was not able to see these in living colonies of 

 V. minor, but could see them very distinctly in a few spore-bearing 

 colonies of V. globr.tor, especially at the anterior end. The hyaline 

 layer is much thicker in V. minor than in V. globator and the zooids 

 are much more nearly spherical in the former than in the latter, 

 in which they are in general quite angular. The difference in the 

 shape of the zooids forms the chief distinguishing characteristic 

 of the two species. The cells in the colonies are arranged side 

 by side so as to form a wall enclosing a cavity. In V. minor the 

 hyaline layer is figured by Meyer ('95, p. 227) as extending nearly 

 to the middle of the colony, thus leaving only a very small central 



