CELL-DIVISION 



equator, whilst others insist that the increase is over the 

 poles. Despite this lack of accord concerning the efficacy 

 of the infallible principle of surface-tension in the division 

 of oil-drops, to say nothing of the cells themselves, the 

 surface-tension theory persists. 



More than once I have protested against the widespread 

 misuse of surface-tension in biological explanations and 

 theories.^ Drops of oil in water or solution constitute a 

 liquid-liquid system; a suspension of eggs does not. Whilst 

 every oil-drop is a homogeneous liquid, each egg, far from 

 being homogeneous, is itself a suspension of oil-drops, yolk- 

 spheres, etc. — of materials of diflferent chemical structure 

 and physical make-up in cytoplasm, the living continuum, 

 itself heterogeneous. The surface of an oil-drop is a film 

 of molecular dimension chemically identical with the inte- 

 rior of the drop; the measurable surface of an egg is thou- 

 sands of times the width of the film of an oil drop and is a 

 diflferentiated structure built by the egg during its develop- 

 ment as a germ cell. The fact that one can induce stream- 

 ing in oil-drops which closely resembles cytoplasmic 

 streaming does not warrant the conclusion that since an oil- 

 drop divides because of changes in surface-tension, division 

 of the egg or other cells is likewise due to changes in surface- 

 tension. Models used in biology to prove the cause of 

 vital phenomena may be interesting but after all they are 

 literally only models — imitations of the real thing and of 

 little value for the analysis of the conditions in living 

 substance. 



Equally weak is the support for the surface-tension theory 

 derived from experiments which demonstrate that during 

 the cleavage-cycle, eggs of sea-urchins reveal a rhythm of 



^ Hercik {igj4) entered a strong -protest against the indiscriminate 

 adducing of surface-tension for explaining biological processes. 

 In his opinion the significance of surface-tension is often over- 

 estimated. 



271 



