SURFACE TENSION AND CELL-DIVISION 241 



There can be but little doubt that the explanation of these 

 facts is as follows : the acid sea-water raises the surface tension 

 at the egg-surface, and tends to make the egg regain its spherical 

 shape. Owing, however, to some force, which elongates the 

 main axis of the egg, equilibrium is reached (during the latter 

 stages of cleavage) when the egg is cylindrical and not spherical.^ 

 It will be noted that the increase in length of the main axis, 

 which was noticed in normal cleavage, is much more obvious 

 in acid sea-water owing to the abolition of the cleavage furrow. 



It has been shown that when the egg is removed from acid 

 to normal sea-water the cleavage furrow reappears at once. 

 The amount to which the cleavage furrow develops depends 

 entirely on the equilibrium between (a) the surface tension 

 at the egg-surface and (fe) some other force within the egg. 

 In acid sea-water the surface tension is high and equilibrium 

 is reached when the egg is a cylinder with hemispherical ends ; 

 in normal sea-water equiUbrium is reached with a well-marked 

 cleavage furrow. Whereas the effect of a change in surface 

 tension is very rapidly reflected in the form of the cleavage 

 furrow, it is also clear that the elongation of the main axis is 

 the active process whereby free energy is supphed to the egg- 

 surface and allows the furrow to form under normal conditions. 

 This process is stopped in an acid solution (like so many other 

 physiological processes) and is resumed on return to normal 

 sea-water. The rate at which this force acts is entirely indepen- 

 dent of the experimental rate of change of the surface tension 

 of the egg-surface. 



That the elongation of the egg axis is the active process 

 involved is shown from the experiment of Plateau.' 



If a drop of oil be placed between two metal rings A and B so 

 as to form a complete cylinder (Text-fig 6, a), and if the rings 

 be now moved apart, then when the distance of A to B becomes 



^ The relative surfaces which enclose an equal volume of protoplasm are 



(i) Sphere 100 



(ii) Cylinder with hemispherical ends . 105 

 (iii) Two spheres (each half vol. of i) . 1^6 

 ' Statique des liquides ', vol. ii. 



