18 ELEMENTARY BIOLOGY. [ill 



dopodia, and therefore forms its hindmost part when it moves. 

 Each pseudopodium is evidently, at first, an extension of the 

 denser clear substance (ectosarc) only ; but as it enlarges, the 

 central, granular, more fluid substance flows into its interior, 

 often with a sudden rush. 



In some Amcebce a clear space makes its appearance, at 

 intervals, in a particular region of the ectosarc, and then 

 disappears by the rapid approach of its walls. After a while, 

 a small clear speck appears at the same spot and slowly 

 dilates until it attains its full size, when it again rapidly 

 disappears as before. Sometimes two or three small clear 

 spots arise close together, and run into one another to form 

 the single large cavity. 



The structure thus described is termed the contractile 

 vesicle or vacuole, and its rhythmical systole and diastole 

 often succeed one another with great regularity. 



Nothing is certainly known respecting its function, nor 

 even whether it does or does not communicate with the 

 exterior, and thus pump water into and out of the body of 

 the Amoeba, though there is some reason to think that this 

 may be the case. 



Very frequently one part of the Amoeba exhibits a 

 rounded or oval body, which is termed the nucleus. This 

 structure sometimes has a distinctly vesicular character, and 

 contains a rounded granule called the nucleolus. 



The gelatinous body of the Amoeba is not bounded by 

 anything that can be properly termed a membrane ; all that 

 can be said is, that its external or limitary layer is of a 

 somewhat different constitution from the rest, so that it 

 acquires a certain appearance of distinctness when it is acted 

 upon by such reagents as acetic acid, or when the animal is 

 killed by raising the temperature to 45 C. Physically, the 

 ectosarc might be compared to the wall of a soap-bubble, 

 which, though fluid, has a certain viscosity, which not only 



