UNICELLULAR FORMS 6i 



or otherwise broken down in cell metabolism, many of the sub- 

 stances formed are dissolved non-gaseous substances which in the 

 pure state are either liquids or solids. These waste products in all 

 cells pass out by diflFusion through the cell wall and in addition in 

 amcebx some are passed out with the water in the contractile 

 vacuole. A sharp distinction must be drawn between this true 

 excretory process in which substances derived from and by the cell 

 metabolism are gotten rid of, and the expulsion from the gastric 

 vacuoles of undigested and indigestible materials, or defecation. 

 In the first case the substances were formerly a part of the proto- 

 plasm; in the second case, the material never was within the cell 

 and is in reality a foreign body. This distinction between true excre- 

 tion and defecation applies not only to amcebx but to all animals. 

 In fact, the two processes are so separate and distinct that quite 

 different mechanisms function to expel the two types of materials. 

 Amoeboid Movement. There have been many and varied 

 attempts to solve the problem of how the amoeba moves and how 

 it engulfs its food. The problem is important for this type of cell 

 locomotion is not confined to the amoeba alone, but is found in inde- 

 pendent cells in most animal bodies. Even in Man one type of cell 

 found in the blood stream (white cells) normally moves in a similar 

 manner. Prominent among the explanations that have been ad- 

 vanced is the surface-tension theory of amoeboid movement. This 

 theory is based on the fact that if the surface of a drop of liquid 

 is weakened at some local area, the tension on the remainder of 

 the drop remaining high, the contents of the drop are thus forced 

 to push out the weakened area. So, a drop of chloroform in water 

 can be made to move about in a manner resembling the motion 

 of the amoeba by the local application of a substance which lowers 

 surface tension (Fig. 14). The theory holds that, either from an 

 external or an internal source, substances efifect local lowering of 

 surface tension on the amoeba and thus the formation of pseudo- 



