372 RALPH S. LILLIE 



judging from the analogy of the stimulation-process, would 

 alter the metabolic processes, e.g., oxidations, within the cell; 

 it would also involve increase of suiface-tension, and under 

 appropriate conditions a definite change of form would result. 

 Evidently this assumed increase of permeability and surface- 

 tension cannot be uniform over the whole cell-surface;^^ in order 

 to account for the definite and symmetrical change of form ob- 

 served in cell-division, it seems necessary to assume that the 

 surface-tension is chiefly increased over two symmetrically placed 

 areas centering at the poles and extending to near the equator; 

 these coincide with the regions of increased permeability and 

 decreased electrical polarization. By the traction of these 

 areas on the surface-protoplasm at the relatively unaltered equa- 

 torial zone, where surface-tension remains low, material is 

 removed from this latter region, with the result that the cleavage- 

 furrow is formed and progressively deepens. This hypothesis 

 regaids the egg as having the properties of a droplet of viscous 

 fluid, and ascribes cytoplasmic division to surface-forces. The 

 assumed distribution of the areas of altered surface-tension 

 would account for the observed change of form;'^ the hypothesis 



1' This is evident, since all freely suspended homogeneous fluid droplets, 

 whatever their surface-tension, have alike the spherical form. Departure from 

 sphericity, under these conditions, means unequal tension at different portions 

 of the surface. A symmetrical change of form, as in the dividing sea ucchin 

 egg, implies symmetrical distribution of the areas of altered surface-tension. 



" Cf. the experiments and discussion of T. B. Robertson, Arch. f. Entwick- 

 lungsmech., 1909, vol. 27, p. 29, and 1913, vol 35, p. 692. Robertson's conclusions 

 arc rejected by McClendon (ibid., 1913, vol. 37, p. 233), on what seem to me in- 

 sufficient grounds. 



The considerations adduced by Donnan (Zeitschr. f. physik. Chem., 1899, 

 vol. 31, p. 42), to explain why adjacent droplets in an emulsion of oil in weak 

 soap solution do not fuse, also apply here. When two such drops accidentally 

 come into contact, union is prevented because their approach increases the 

 concentration of soap at the region of contact and there lowers the surface-ten- 

 sion; this causes a mutual withdrawal of the drops through the higher tension 

 of the rest of the surface; i.e., the contiguous portions of the surfaces are drawn 

 apart by the greater traction exerted by the non-contiguous portions. Similar 

 reasoning will apply to a partly divided cell. See Donnan's diagram, p. 48 

 (also given in Freundlich's Plapillarchemie, p. 456). According to the present 

 conception, the separation of the two blastomeres in the dividing egg is similarly 

 due to the greater surfa:ce-tension of the circumpolar and 'temperate' zones of 

 the cell-surface as compared with the equatorial zone. 



