BINARY FISSION AND SURFACE TENSION IN THE 

 DEVELOPMENT OF THE COLONY IN VOLVOX 



R. A. HARPER 



Columbia University 



In Volvox we have an incipient metaphyte with a many-celled body 

 of definitely organized form and the fundamental differentiation of 

 soma and germ cells fully established. Klein's ('89, '90) argument 

 that Volvox differs from true metaphytes in that the differentiation 

 of the germ cells does not take place until after cell division is com- 

 pleted is not very illuminating. The germ cells do not appear in 

 Oedogonium until a considerable series of undifferentiated cells have 

 been formed. Falkenberg's comparison of the entire colony to a 

 zoosporangium also overlooks the very important fact that in the 

 development of the Volvox colony growth regularly alternates with 

 cell division (at least after the first few divisions) just as it does in 

 the development of the soma of one of the higher metaphytes. Vol- 

 vox is frequently referred to as one of the best known algae. There is 

 general agreement as to the order of cell divisions in the formation of 

 the colonies, both for the egg and the asexual germ cells. The litera- 

 ture has been frequently summarized. 



To proceed further with the study of Volvox from the standpoint 

 of evolution and morphogenesis, we may compare it with such a 

 simple coenobe as Goniiim. In addition to their more obvious struc- 

 tural characteristics there are two essential differences between the 

 colonies in such forms as Goniiim and Volvox. First, the adhesion 

 between the daughter cells is much more firm in the latter. In 

 Gonium, as all observers testify, the adult colonies break up into their 

 component cells with the greatest readiness. I have figured such 

 broken-up colonies ('12, PI. V, Fig. 23). Slight pressure, change in 

 the chemical composition of the medium in which they are, etc., lead 

 to almost explosive separations. One marked difficulty in getting 

 good photographs of the colonies is due to their tendency to go to 

 pieces. On the other hand, Volvox shows almost no tendency to 

 separate into its component cells. The colonies may be crushed into 

 formless masses without isolating a single cell and I know of no chemi- 

 cal or other stimulus which will cause their cells to fly apart as do 

 those of Goniiim. 



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