366 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 3 



meability of the cell wall to the substances provided. If the latter 

 are of a kind that can penetrate the retaining membrane, they will 

 enter the cell substance, whether they be foods or poisons. It be- 

 hooves an animal, therefore, to beware of what it puts into its 

 stomach. One reason why substances in solution go through cell 

 walls, or through any animal membranes, if they can, is the same 

 as that which makes gas molecules in air, or " solid " molecules in 

 water, spread all through the space available to them. You cannot 

 put a gas in one corner of a room or salt in one side of a pan of 

 water and make it stay there. The activities of the molecules make 

 them go to the places where molecules of their own kind are scarce 

 until there is an even distribution all through the medium. A per- 

 meable partition in the latter makes no difference in the final result. 

 On this principle of dljfusion^ waste products, with which the cells 

 become supercharged, pass out through the cell wall, and building 



'"'b '"' C~' 



Figure 2. — Diagrams showing the method of feeding by direct ingestion of food material 



as practiced by an amoeba. 



materials, of which there is a deficit in the cells, pass in if they are 

 present in the cell environment. The laws of ordinary diffusion, 

 however, do not account for all the phenomena of diffusion though 

 cell walls. Some cells of the animal body have the power of con- 

 verting food substances into a form that will pass the cell wall; 

 others have not this power and must have their food prepared for 

 them. 



Materials containing the essentials of animal food are abundant 

 almost everywhere in nature, but unfortunately most foodstuffs in 

 their native state are not the kind of things that will dissolve in 

 water and that will go through the semipermeable walls of animal 

 cells. The first ancestors of all living cells, therefore, must have 

 been confronted with the problem of devising a means to keep from 

 starving in the midst of plenty. 



Some of the one-celled animals, such as the amoeba, have solved 

 this problem of feeding in a very simple manner, without reference 

 to the permeability of the cell wall. The amoeba (fig. 2) being a 

 soft and plastic creature, simply flows around any mass of food 

 material in its course (B) until the food is pressed through its 



