236 J. GRAY 



of many other cells, in that it is essentially a liquid which is 

 immiscible with water. Further, an overwhelming body of 

 evidence is available to show that the protoplasmic surface 

 contains a lipoid or oily phase. To what extent is the form of 

 the egg dependent upon those conditions which determine the 

 form of inert drops of oil surrounded by water ? 



Consider the simple case of oil-drops in water. The drops 

 are all spherical owing to the existence of a force — usually 

 called surface tension — acting at the oil/water interface. 

 In any such system the amount of free energy will tend to reach 

 a minimum, and since the volume of oil presents a minimum 

 amount of surface when the drop is spherical it is obvious that 



Text-fig. 1. 



Form of oil-drops in («) acid water (6-f?) increasing amounts of 



alkali in water. 



the position of stability is reached when the drops are round. 

 The higher the surface tension the more rapidly is the spherical 

 form assumed, and the more resistant is the form of the drop 

 to external or internal disturbance. Now. it has been shown 

 that the surface tension at an oil/ water interface is materially 

 affected by the hydrogen-ion concentration of the water. 

 Hydrogen ions raise the surface tension ; hydroxyl ions lower 

 it. The effect of such changes is a marked alteration in the 

 form of the oil. 



In an acid solution the oil- drop remains perfectly spherical 

 and is not readily deformed by external forces. In an alkahne 

 solution, however, the drop becomes very irregular in shape 

 and is readily deformed. In highly alkaline solutions the sur- 

 face tension may actually become ' negative ', and the condition 

 of stability is reached by the splitting up of the drop to form 

 an emulsion. 



