6 GENERAL ZOOLOGY 



protoplasm and which form part of the picture of the anatomy of 

 some animals. 



A small mass of living protoplasm viewed through the micro- 

 scope appears as a faintly grayish, jellylike substance in which 

 granules are suspended. Sometimes a streaming or flowing 

 movement of materials can be seen. Protoplasm is heavier than 

 water and somewhat more refractive to light. 



Protoplasm has a semifluid consistency which is known as a 

 colloidal state of matter. This means that some of its constit- 

 uents at least, instead of being in the form of a true solution, 

 consist of suspensions of molecular aggregates dispersed in a 

 medium. The medium in this case is largely water. In a true 

 solution these aggregates of molecules would separate as indi- 

 vidual molecules which in turn might dissociate into ions. The 

 molecular aggregates are therefore much larger than the particles 

 making up the solute in an ordinary solution. Their size varies 

 roughly between 0.0001 and 0.000001 mm. in diameter. In 

 protoplasm there seem to be three physical systems, viz., true 

 solutions, emulsions, and gels, of which the latter two are colloidal 

 systems. 



The colloidal state is common enough in nonliving nature 

 where the components may be solid, liquid, or gaseous, in various 

 combinations. Thus smoke is a colloidal mixture of solid in a 

 gas; foam, of gas in a liquid; fog, of liquid in a gas, etc. Proto- 

 plasm seems to be largely an emulsion, i.e., the components con- 

 sist of liquid phases, such for example as is produced when oil is 

 shaken up in water. In an oil-and-water mixture the components 

 usually separate in the course of time, if undisturbed, unless some 

 other substance is added that serves to keep one liquid phase 

 dispersed in the other. This is accomplished in the preparation 

 of mayonnaise dressing by adding yolk of egg to an emulsion of 

 oil and water (vinegar). The yolk of the egg serves to preserve 

 the colloidal state of the oil in the aqueous medium by acting as a 

 binder. The physical state of protoplasm is roughly approxi- 

 mated by the physical state of mayonnaise dressing. 



Substances in the colloidal state diffuse slowly or not at all 

 through animal membranes, which partially explains why living 

 protoplasm does not dissolve in water. Colloids within limits 

 have the power of changing from a fluid or sol state to a more 

 solid or gel state and back again. 



