PROTOPLASM 47 



That protoplasm is a colloidal system is indicated by many of its 

 characteristics. Like other colloids it differs from true solutions in its 

 manner of flow, in the relation of its viscosity to stress, and in behavior 

 involving surface tension, adsorption, and permeability. Its physical 

 consistency varies widely during certain processes such as cell division 

 and is strongly affected b.v stimuli of various kinds. It even exceeds 

 some other colloidal systems in its resistance to separation of phases by 

 centrifugal forces: after being centrifuged for 1 hour at 400,000 times 

 gravity, Ascaris eggs recovered and divided, and cleavage actually 

 occurred during centrifugation at 100,000 times gravity. 



It is not known at present what type or tj'pes of colloidal structure are 

 characteristic or essential in protoplasm. Although protoplasm fre- 

 quently shows within the visible range a structure like that of an emulsion, 

 it is uncertain how far such a structure continues into the submicroscopic 

 range. As a matter of fact there are rather definite indications that the 

 structure is not primarily of the emulsoid type: protoplasm is elastic, 

 emulsions are not; protoplasm has a limited, though high, imbibition 

 limit, whereas emulsions do not; protoplasm coagulates, emulsions do not 

 (coagulation of milk involves the protein, casein, not the fat forming the 

 visible emulsion) ; across the plasma membrane is a continuous water 

 path, and if the other phase or phases were discontinuous the membrane 

 would disintegrate in water. Such considerations point to the existence 

 of some sort of structural framework not capable of indefinite dispersion 

 like an emulsion. 



The view that protoplasm has an important fibrous element in its- 

 structure along with nonfibrous constituents has recently gained strong 

 support. The "fibrillar theorj^" of many years ago was based largely 

 upon what was directly seen in living and fixed cells. Our modern 

 interpretation has come not only from investigations of protoplasm with a 

 variety of new techniques, but also from physical and chemical studies on 

 inorganic colloidal systems, on certain products of biological activity, and 

 especially on the proteins. The "fibers" that now concern us are mainly 

 something far smaller than the workers of half a century ago had in 

 mind. 



Especially instructive are the results of researches on the structure of 

 inorganic systems, notably those formed by vanadium pentoxide, zinc 

 oxide, and silicon hydroxide in water. In polarized light between crossed 

 nicols, these su))stances wken flowing in narrow channels show double 

 refraction of a type that indicates the presence of minute linear elements 

 lying parallel; moreover, such an arrangement is sometimes assumed 

 spontaneously without flow. The linear elements here are evidently 

 chains of elongate molecules: in the silica gel, for example, the Si(OH).j 

 molecules join end-to-end, losing HoO at each junction, to form long 



