1 8 ELEMENTARY BIOLOGY. [CHAP. 



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 sub- 

 stance flows into its interior, often with a sudden rush. 



In some Amoeba 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. Physic- 

 ally, the ectosarc might be compared to the wall of a 

 soap-bubble, which, though fluid, has a certain viscosity, 

 which not only enables its particles to hold together and 



