AMEBA 39 



large granules. No fixed line of separation between it and the 

 ectoplasm is possible. 



The nucleus (Fig. 9, i) is not easily seen in living specimens. 

 In animals that have been properly killed and stained it appears as 

 a spherical body lying in the endoplasm. Its position is not 

 definite, but changes during the movements of the Ameba. It has 

 a firm membrane and contains a great many spherical particles of 

 chromatin scattered about in the nuclear sap. During the life 

 of Ameba, before the period of reproduction, the nucleus plays 

 an important role in the metabolic activity of the cell. This has 

 been proved by experiments in which the animal was cut in two. 

 Invariably the vital processes were disturbed in the enucleated 

 fragment, and death resulted. Profound changes in the nucleus 

 take place during reproduction; these will be described in detail 

 later. 



The contractile vacuole (Fig. 9, 2] is a clear space filled with a 

 fluid less dense than the surrounding protoplasm. It derives its 

 name from the fact that at more or less regular intervals it sud- 

 denly disappears, its walls having contracted, thus forcing out 

 the contents. That the vacuole discharges to the outside of the 

 body has not been definitely observed in Ameba, no doubt because 

 the fluid is usually expelled on the upper surface of the body and 

 therefore cannot be seen (38). " At first the vacuole lies near 

 the nucleus, but as it grows, it becomes separated from the latter, 

 and at the time of its contraction lies at the end of the body far- 

 thest from the advancing pseudopodia, at what is sometimes called 

 the posterior end. Its reappearance is always somewhere near 

 its point of disappearance. While still small it is carried along 

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

 where it completes its development " (29, p. 88, Fig. 10). 



The functions of the contractile vacuole are excretory and res- 

 piratory, as is explained on page 50. Several investigators have 

 recently regarded it as a hydrostatic organ, regulating the quantity 

 of water contained in the body of the animal, and so its weight. 

 This would also afford a means of getting rid of the water taken 



