8 THE CELL [CH. 



practically structureless. The first of trie three hypotheses 

 is that protoplasm consists of a relatively solid network or 

 reticulum enclosed in a fluid, like a sponge in water; this 

 may be called the reticular hypothesis of FLEMMING. The 

 second, or alveolar hypothesis, of Birrs CH LI, regards the 

 more solid portion as forming a sort of foam, enclosing the 

 fluid in the cavities or alveoli. In thin sections, of course, 

 the alveolar and reticular structures would give similar ap- 

 pearances, but they differ in the fact that in a reticulum 

 the fluid is continuous, while in a foam structure the 

 cavities are closed so that the fluid is divided into a number 

 of separate compartments. The third hypothesis is that of 

 ALTMANN, which regards the more solid portion as having 

 the form of granules, suspended as in an emulsion in a 

 fluid medium. Each of these three hypotheses is in agree- 

 ment with the facts, in the sense that either reticular, 

 alveolar or granular structure may be obtained in fixed pre- 

 parations of protoplasm, according to the material chosen 

 and the fixative employed. But since each type of structure 

 may be obtained in apparently well-preserved material, 

 there is no satisfactory means of choosing between them, 

 since the structure is too fine for definite results to be ob- 

 tained from an examination of living protoplasm. Under 

 these circumstances the doubt naturally arises whether 

 any of the three represents the real condition as it exists in 

 life, and whether the apparent structures seen in fixed 

 material may not be due to the process of coagulation 

 under the influence of different fixatives. The probability 

 that this is so is greatly increased by the results obtained 

 by HARDY (1899) in the.study of the action of different fixa- 

 tives on a variety of colloidal solutions. HARDY found that 

 when a solution of a colloidal substance is caused to coagu- 

 late, the more solid portion separates from the fluid with 



