30 NORMAL HISTOLOGY. 



by the streaming of the cell-body into that protrusion. The 

 fate of the particle thus incorporated depends upon its nature. 

 If it be serviceable as food, it is gradually digested and ab- 

 sorbed, or such parts of it as are digestible are so utilized, and 

 the remainder, no longer of use to the amoeba, is extruded from 

 its body. 



These phenomena reveal powers of perception and selection on 

 the part of this cell which are very closely akin to the intelligence 

 of more complex organisms. They also demonstrate its power of 

 assimilating material from without, to serve as nourishment and the 

 source of the energy which it expends in executing its movements 

 and in carrying on the chemical processes pertaining to its internal 

 economy. 



At intervals, there appears within the endoplasm a small, clear, 

 spherical spot. This gradually increases in size and constitutes a 

 little drop of fluid, sharply defined from the surrounding cytoplasm. 

 After it has attained a certain size, it suddenly disappears, the cyto- 

 plasm around it coalescing and leaving no trace of its existence. 

 Such a clear space, filled with fluid, within the body of a cell is 

 called a vacuole, and those which are suddenly obliterated, contrac- 

 tile vacuoles. Their purpose is not clearly understood, but prob- 

 ably has to do with a primitive circulatory or respiratory function, 

 since contractile vacuoles are not observed in the cells of higher 

 organisms where those functions are carried on by more elaborate 

 mechanisms. 



Eventually the amoeba reproduces its kind by dividing into two 

 similar cells, each of which grows into a likeness to the parent 

 individual. 



Let us now compare the amoeba with some other varieties of cell, 

 in order to learn what they all have in common. 



The amoeba has an outer, soft, transparent layer of cytoplasm, 

 the ectoplasm. This is not present in all cells. In many the 

 granular cytoplasm has no envelope, but appears to be quite naked. 

 In other varieties it is enclosed in a distinct membrane. 



In the great majority of cells the active streaming of the cyto- 

 plasm and the pseudopodial protrusions described in the amoeba are 

 wanting, but the Brownian movement of the granules is more con- 

 stantly present. The cells have fixed positions and their food is 

 brought to them, usually in solution, so that the more active move- 

 ments so essential to the welfare of the amoeba would be superfluous. 



