122 GENERAL BIOLOGY 



has a rather firm nuclear wall or membrane and there are quite a num- 

 ber of spherical particles of chromatin scattered about in the nuclear 

 sap. The contractile 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, or, what is commonly called 

 the posterior end. It then re-appears close to the point of its disappear- 

 ance, being carried along by the streaming protoplasm back to a posi- 

 tion near the nucleus, again passing through the same stages just de- 

 scribed. <- 



The fluid content of the contractile vacuole contains urea. As this 

 is the common excretory substance of all animals we know that the 

 contractile vacuole is excretory. It is also respiratory because COX in 

 all probability passes to the exterior of the body by way of this organ, 

 the oxygen itself being taken in through the outer surface of the body. 

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

 respiration of a h'igher animal, such as the frog. The food vacuoles come 

 into existence 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 out-pushings are 

 known as pseudo-pods ( ' ), or rhizopods ( ). 



Often there are several of these pseudopods thrust out at one time, al- 

 though it is usually the one which comes in contact with some object 

 which gains the mastery, all of the animal then rolling forward so that 

 the cytoplasm extends into the out-pushing mentioned. 



There have been various theories advanced to account for their 

 movements (Fig. 45), they being as follows: 



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

 water or any inorganic liquid be 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. There are many pseudopods which are 

 extended out into the surrounding liquid, however, and do not come in 

 contact with any other solid substance; so, 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 the 

 study of physics and chemistry and supposes that ther.e are various cur- 

 rents which move forward or outward in the central axis and backward 

 along the surface. Unfortunately for this theory, the currents in the 

 amoeba do not run that way. 



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



