PROTOPLASM 37 



are like those of other known colloids. Its semipermeable properties are 

 typically those of colloidal systems; a semipermeable region is probably 

 present wherever protoplasm comes in contact with other substances, 

 such as water. Protoplasm shows most strikingly its colloidal character 

 in the alterations of physical state, involving imperfectly understood 

 structural changes, which it undergoes as a result of variations in external 

 conditions and internal reactions. Local temporary alterations of this 

 nature are known to accompany a number of important life processes. 

 Protoplasm, like many organic and inorganic colloids, is irreversibly 

 coagulated by too high temperatures and a variety of chemical substances. 

 The "fixation" of protoplasm by the reagents employed in cytological 

 technique is primarily the transformation of a lyophilic system into an 

 irreversible lyophobic coagulum. 



Protoplasm is colloidal, but the question of the particular type or 

 types of colloidal structure it possesses is one to which no satisfactory 

 answer can yet be given. The most widely prevalent theory is that the 

 emulsion type of structure, often visible with the microscope, is continu- 

 ous with an ultramicroscopic colloidal structure of the same sort, with 

 discontinuous phases of lipides, proteins, and other liquid and solid sub- 

 stances suspended in an aqueous medium. It has been thought by some 

 that high electrical resistance indicates a discontinuity of the water phase 

 (M. Fischer, 1923), some non-conducting material constituting the medium 

 of dispersion. The suddenness of many permeability changes has been 

 thought to favor the view that protoplasm has an emulsion structure, at 

 least in plasma membranes (Clowes, 1916). R. S. Lillie (1923) sees 

 further evidence in a variety of phenomena, including the autolytic action 

 of injured cells wherein enzyme and substrate are allowed to interact by 

 the breakdown of films normally separating them. 



The view that the submicroscopic colloidal constitution of protoplasm 

 is primarily like that of an emulsion has been brought into question by 

 Seifriz (1924a6, 1926a6c, 19295). He points out that living protoplasm, 

 although it may show such a structure within the power of the micro- 

 scope (Fig. 16), differs markedly from emulsions in its noticeable degree of 

 elasticity, its power of imbibition, and its characteristic behavior in form- 

 ing a granular irreversible coagulum at death. Comparisons wdth certain 

 inorganic systems suggest that it has a structure in which the units are 

 arranged in linear chains which may form a three-dimensional network or 

 "brush heap." 



It is not to be concluded that investigators insist that all protoplasm 

 must have this or that particular type of colloidal structure. It is 

 probable that the minute structure varies within wide limits in different 

 tissues and in different regions of any differentiated mass of protoplasm. 

 It may be that emulsions, interlacing systems, and molecular bridgeworks 

 all actually exist and pass one into another according to general and local 



