88 THE PROTOZOA 



Amoeba proteus, with its comparatively clear protoplasm arid free- 

 dom from pigments, is one of the most favorable objects for the study 

 of the contractile vacuole. If a sufficiently high power is used, the 

 formation and contraction of the vacuole and the expulsion of the con- 

 tents to the exterior can be followed step by step. 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 

 farthest from the advancing pseudopodia, at what is sometimes called 

 the posterior end (Fig. 49, F). 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. The increasing weight of the 

 growing vacuole causes it to lag behind the streaming granules and 

 nucleus, until at its full growth it is widely separated from the latter 

 organ. The vacuole may appear to move in the direction contrary to 

 that of the protoplasmic streaming, although in reality it is quiescent; 

 for while it remains in the field of the microscope, the main body of 

 the animal moves well out of it, until the vacuole is surrounded only 

 by the posterior end of the animal (G), which is reduced to a thin 

 layer of granules and a hyaline layer of ectoplasm between the vacu- 

 ole and the exterior. The granules later move away, passing around 

 the vacuole, until finally there is only a thin layer of hyaline 

 plasm between the vesicle and the exterior. Shortly after this the 

 vacuole bursts and disappears, in most cases a distinct bulge toward 

 the outside preceding contraction. Contraction always begins on 

 one side of the vacuole, and is carried across it toward the outer 

 edge (H). 



Stokes ('93) asserts that there is no bursting of the wall, but that 

 minute pores are formed through which the contents of the vacuole 

 are forced to the outside. In some instances the contents of the 

 vacuole are not completely emptied, but as much as half may be left, 

 the vacuole then rounding out to be carried back by the streaming 

 plasm to the nucleus, where it completes its growth. In other cases 

 the contents of the old vacuole may be entirely discharged, with the 

 exception of a small quantity of liquid retained in exceedingly small 

 vesicles, each of which may grow to some extent independently, 

 although they ultimately fuse to form the new vacuole (_/). Thus 

 the new vacuole does not necessarily re-form at the place of dis- 

 appearance, but may be derived by the coalescence of a number 

 of smaller ones which themselves are the remains of an old one. 

 As many as six may unite in this manner to form the new vacuole 

 {A, B, C.) These unite two by two in various parts of the plasm, 

 and the last two may not fuse until in the neighborhood of the 

 nucleus. 



