68 PROTOPLASMIC ACTION AND NERVOUS ACTION 



ties of living matter have arisen — or which has made it 

 possible for systems with vital properties to evolve — 

 is that of an emulsion; i.e., a polyphasic system with thin 

 interfacial films separating two or more component 

 phases which have fluid or solvent properties. From 

 general considerations it seems clear that some kind of 

 pol}^hasic structure must be assumed in order to account 

 for such a universal property as that of growth; the unit 

 of living matter, even while it continues to increase in 

 size, retains a complex and specific composition different 

 from that of the surroundings; and this peculiarity is in 

 itself incompatible with structural homogeneity, since 

 the elementary need of providing against free diffusive 

 interchange with the surroundings requires a surface 

 layer with properties different from those of the internal 

 protoplasm. This must be true even of the simplest 

 forms of living matter. We cannot compare the proto- 

 plasm of ultra-microscopic organisms with self-propa- 

 gating enzyme-like material (supposing such material 

 possible), as has been done, since the physical con- 

 ditions necessary for metabolism and growth must 

 exist in even the simplest living systems; and this 

 requires at the very least a differentiation between the 

 more permanent or solid components of the system and 

 the liquid components which contain in solution simpler 

 materials (nutrients and oxygen) which are continually 

 being renewed. 



It has long been recognized that colloids form the 

 basis of protoplasmic structure. Hardy's investigations 

 showed that many of the characteristic structural 

 appearances presented by fixed and stained protoplasm 

 in microscopic preparations were incidental consequences 



