LIVING SUBSTANCE 15 



directions. The colloidal nature of the proteins, 

 therefore, is probably to be attributed to the great 

 size of the molecules of which they are composed, 

 aggregates which in fact are not true molecules but 

 composites of other smaller aggregates. Chemists 

 refer to this welding together of molecular aggre- 

 gates as polymerization. As we shall see later, the 

 process of animal digestion involves merely the 

 breaking up of these aggregates into others of lesser 

 degree, small enough to diffuse through the lining 

 membranes of the alimentary canal. 



Protoplasm, being composed largely of proteins, 

 is thus colloidal in its physical make-up. But the 

 examination of living protoplasm with the high 

 powers of the microscope reveals a structure much 

 more complex than may be found in a mere lump 

 of non-living colloid. Living substance has a 

 characteristic physical structure of its own, to ex- 

 plain which several theories have been advanced. 

 According to one, the essential basis of protoplasmic 

 structure is granular, and granules are certainly to 

 be found in protoplasm. Others find fibrils like 

 detached threads, others see a skein or reticulum in 

 the meshes of which more watery substances are 

 held. The view first advocated by Butschli is, 

 however, the one most commonly held by biologists. 

 According to this theory, protoplasm has the struc- 

 ture of a foam in which the denser parts surround 

 the lighter as the film of water does the air in soap 

 bubbles. Perhaps a more accurate object of com- 

 parison would be a fine emulsion. In an emulsion, 



