The Protozoa 123 



The nucleus changes with the various movements of the animal, so 

 that it will not be found in the same location in all Amoebae. It has a 

 rather firm nuclear wall, or membrane, and quite a number of spherical 

 particles of chromatin are scattered about in the nuclear sap. The con- 

 tractile vacuole usually lies near the nucleus, but as the vacuole grows it 

 becomes further and further separated from the latter, and by the time 

 it is ready to contract and expel its contents, lies close to the end farthest 

 from the pseudopodia, at what is commonly called the posterior end. 

 It then reappears close to the point of its disappearance, being carried 

 along by the streaming protoplasm back to a position near the nucleus, 

 again passing through the same stages just described. 



The fluid content of the contractile vacuole is believed to contain 

 urea. As this is the common excretory substance of animals, the vacuole 

 is probably excretory in function. It is likely that it is also respiratory 

 for, in all probability, C0 2 passes to the exterior of the body from the 

 vacuole. Oxygen is taken in through the outer surface of the body. It 

 is well to compare Amoeba's physiological functions with the respiration 

 of a higher animal such as the frog. The food vacuoles come into exist- 

 ence whenever food is taken into the organism, each vacuole seemingly 

 acting as a temporary stomach. 



MOVEMENT 



The ectosarc, also called ectoplasm, sends out finger-like projections 

 into which the cytoplasm of the cell then flows. These outpushings are 

 known as pseudopods ( ), or rhizopods ( ). 



Often several of these pseudopods are thrust out at one time, although 

 usually the one which comes in contact with some object gains the 

 mastery, all of the animal then moving forward so that the cytoplasm 

 extends into the outpushing. 



Various theories have been advanced to account for Amoebae's 

 movements (Fig. 45), as follows: 



1. The adherence theory. This merely means that, if a drop of 

 water or any inorganic liquid is placed upon a flat surface, a part of it 

 coming in contact with some other substance, the entire drop will gravi- 

 tate toward the attached end. Many pseudopods extend out into the 

 surrounding liquid, however, and do not come in contact with any other 

 solid substance. While this theory might explain those pseudopods 

 which do become attached, it does not explain those which are known 

 as free and which do not come in contact with solid objects. 



2. The surface tension theory. This theory is also taken from 

 physics and chemistry and supposes that various currents move forward 

 or outward in the central axis and backward along the surface. Unfor- 

 tunately for this theory, the currents in Amoebae do not run that way. 



3. The contractile theory. 1 This theory has had a varying history, 



*It should be noted that this theory, even if true, in reality explains nothing. It simply pushes 

 the problem back one step farther. That is, if it should prove true that Amoebae contain a con- 

 tractile substance by virtue of whose properties they move, it would then be necessary to explain 

 contractility in the substance — whence the property came, and what its actual meaning may be. In 

 other words, we are forced back to a consideration of the fundamental physiology of the cell. 



