30 NORMAL HISTOLOGY. 



hilar movements is called geotropism and may have either sign. Cells 

 may also either seek or be repelled by moisture (hydrotropism), or by 

 electric currents (galvanotropism). If a pseudopodium comes in con- 

 tad with some particle in the surrounding medium, it may retreat from 

 it, appeal- indifferent to it, or he attracted and proceed to incorporate 

 it. This is accomplished by the cytoplasm flowing around the for- 

 eign body and coalescing on its further side so as to enclose it. It 

 is then conveyed to the body of the cell, either by cytoplasmic cur- 

 rents, by the withdrawal of the pseudopodium containing it, or 

 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, rilled 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. Many unicellular organisms possess contractile vacu- 

 ole.-, l)in it i- by no means essential or characteristic of the protozoa; 

 for the majority of species composing that group of organisms are 

 devoid of such vacuoles. 



Eventually the amoeba reproduces its kind by dividing into two 

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

 individual. 



