THE STRUCTURE OF PROTOPLASM 239 



under the head of granules, which inchide particles, vacuoles, 

 plastids, secretory granules, mitochondria (spheres, rods, and 

 threads), chromatin granules, globules of oil and yolk, crystals 

 of salt and sugar, grains of starch, etc. Some of these are 

 probably of vital importance; others serve as food; and still 

 others may be waste matter. As a general theory of proto- 

 plasmic structure, the granular hypothesis is not a satisfactory 

 one upon which to interpret cell behavior. The reasons for this 

 will appear as other structural theories are discussed. 



The Emulsion Hypothesis. — There is no doubt but that proto- 

 plasm, superficially viewed, is an emulsion. Doubt exists only 

 as to the function and fundamental nature of this emulsion. 

 The emulsion hypothesis got a firm hold in biology because 

 protoplasm as seen through the microscope is quite evidently 

 an emulsion, and because the colloidal structure of jellies was 

 once thought to be a fine emulsion (the latter idea gave rise to 

 the misleading word "emulsoid"). It was very natural, there- 

 fore, for biologists to assume that the ultimate and hidden 

 structure of protoplasm is an emulsion, just like the coarser and 

 visible one, only finer. Let us first recall what happened to the 

 emulsion hypothesis of the structure of jellies. Donnan and 

 Ellis found that fine and pure emulsions are "model suspension 

 colloids" and not of the jelly type at all. Hatschek, in a search 

 for a possible mechanism in emulsions which would explain such 

 gel properties as elasticity, analyzed the situation mathematically 

 and concluded that "the theory that gels consist of two liquids 

 must be pronounced untenable." 



A substantial support of the emulsion hypothesis of proto- 

 plasmic structure came from Clowes, who evolved an ingenious 

 theory of protoplasmic permeability (page 287). He assumed 

 that the outer layer of protoplasm is a fine (ultramicroscopic) 

 emulsion near the reversal point. When the emulsion swings 

 slightly to one side or the other, it becomes more or less permeable 

 to water-soluble substances — more so when an oil-in-water 

 emulsion and less so when of the water-in-oil type. The hypoth- 

 esis nicely explains certain features of the permeability of 

 protoplasm. Particularly convincing is Clowe's discovery that 

 the proportion of sodium and calcium that keeps an emulsion 

 at the reversal point is exactly that which exists in sea water, 

 blood, and other physiological solutions. Such and other 



