AMEBA 41 



in by the protoplasm through the body wall and consequently 

 regulating the tension between the protoplasm and the surround- 

 ing water. Death by diffluence is thus prevented. 



Food vacuoles (Fig. 9, 4) are produced when food is taken into 

 the body. Such a vacuole has as its nucleus a particle of nutri- 

 tive material and is a sort of temporary stomach. 



Locomotion. Although extremely simple in structure, Ameba 

 carries on practically all of the vital activities characteristic of 

 the higher animals. It is capable of automatic movement, of 

 reacting to various stimuli, of carrying on metabolic processes, 

 of growth, and of reproduction. These are all fundamental 

 properties of protoplasm (Chap. II, p. 10) and are here ex- 

 hibited in their simplest form. 



The locomotive function is located in the ectoplasm. The 

 motile organs are finger-like protrusions of cytoplasm called 

 pseudo podia. A pseudopodium is formed in the following man- 

 ner. The ectoplasm bulges out and enlarges until a blunt pro- 

 jection is produced; the endoplasm then flows into it. The 

 result is a movement of the entire animal in the direction of the 

 pseudopodium. If more than one is formed at the same time, 

 there occurs a struggle for supremacy until finally one survives 

 while the others flow back and gradually disappear. Ameba 

 moves, therefore, by thrusting out pseudopodia and then flowing 

 into them. It is necessary to distinguish between pseudopodia 

 that adhere to the surface of some object and those that are 

 thrust out freely into the surrounding fluid, since an hypothesis 

 that explains the formation of one does not always account for 

 the other. 



There are three principal theories which attempt to explain 

 the formation of pseudopodia; these may be outlined briefly as 

 follows : 



(i) The Adherence Theory. It was long ago discovered that 

 if a drop of inorganic fluid which is spreading on a solid surface 

 is made to adhere on one side more strongly than the other, it 

 will move toward the former side. This fact suggested that the 



