Surface Tension and Cell-Division. 



By 

 J. Gray, M.A., 



Fellow of King's College, Cambridge. 

 With 9 Text-figures. 



The series of changes which a dividing cell exhibits has long 

 suggested to biologists that surface tension plays a dominating 

 role in the process of cleavage. Without exception, theories 

 based on this suggestion have postulated regions of differential 

 tension on the cell-surface ; the surface tension at the equator of 

 the cell has been held to be either higher or lower than that at 

 the polar regions of the cell. Such theories have proved of 

 but little value as a means of further investigation, since there 

 is no apparent means of determining how such a state of 

 affairs could arise, nor is there any apparent differentiation 

 in the microscopical structure near the equator of the cell- 

 surface. 



The evidence here presented suggests that regions of dif- 

 ferential surface tension are unnecessary assumptions, and that 

 cell-division does not take place owing to a change in surface 

 tension at the cell-surface, but owing to a force inside the cell 

 which operates against the surface tension. It is the equihbrium 

 between this force and the normal surface tension which deter- 

 mines the shape of the dividing cell. 



The fertilized eggs of Echinus miliaris form very 

 satisfactory material for a study of cell-division, since the 

 protoplasmic surface of the egg is in direct contact with an 

 aqueous medium and because the actual process of cell-division 

 can readily be followed under the low powers of the microscope. 

 The normal egg is spherical, and if it be crushed or broken the 

 resultant portions show no tendency to mix with the water, 

 but rapidly acquire a more or less spherical shape. From this 

 we may infer that the protoplasm of the egg resembles that 



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