30 GENERAL BIOLOGY 



egested. The digested material is later built up into 

 the substance of the cell protoplasm by the process 

 of assimilation. These three functions, ingestion, 

 digestion, and egestion, like that of locomotion, 

 are not localized, but may take place at any part of 

 the body. 



If bacteria 1 be introduced into the body of an 

 animal at any point, the leucocytes will soon be 

 found gathered in great numbers at the same poinjx 

 It has been shown that this gathering is due to a 

 mechanical, i.e. not purposive, " attraction " exerted 

 on the cells by the chemical substances produced 

 and excreted by the bacteria. In other words, the 

 gathering of the former is a response to an altered 

 condition of the medium in which they exist. In 

 the same way the cells will move from a cool region 

 to one of greater warmth, and on a culture-slide, 

 through which runs a weak current of electricity, 

 may be caused to gather at the negative pole. A 

 dead leucocyte will not respond to any of these 

 " stimuli." The response is therefore a function 

 of living matter and is spoken of as a result of its 

 irritability. Irritability has been defined as " the 

 capacity of living substance of reacting to changes of 

 environment by changes in the equilibrium of its 

 matter and its energy." 



All of the above phenomena may be observed also 

 in Amoeba proteus, a free-living cell found in slime 

 and stagnant water. Amoeba is an independent 

 organism, whereas the leucocyte is one cell out of a 



1 e.g. pus-forming staphylococci. 



