14 ZOOLOGY SECT. 



food being ingested than is required for simply maintaining the 

 size unaltered and soon a remarkable change takes place. The 

 processes become withdrawn, and a fissure appears dividing the 

 Amoeba into two parts (Fig. 2). This fissure grows inwards, and the 

 two parts become more and more completely separated from one 

 another, till eventually the separation becomes complete, and we 

 have two distinct Amoebae resulting from the division of the one. 

 While the protoplasm has been undergoing this division into 

 halves the nucleus has also divided, and each of the two new 

 Amoebae possesses a nucleus similar to the original one, and 

 developed from it by division. It is mainly by this simple process 

 of division into two, or binary fission as it is called, that rcpro- 

 duction or multiplication takes place in the Amoeba. 



In spite of the great simplicity of its structure, the Amoeba 

 thus carries on a number of different functions. The practically 

 structureless particle of protoplasm is able to act on matter 

 absorbed as food in such a way as to alter the chemical composition 

 of the latter and to assimilate it ; it is able to carry on movements 

 of locomotion, as well as movements those involved in the 

 taking in of food particles which may be looked upon as move- 

 -raents of prehension ; it exhibits a certain degree of sensitiveness 

 or irritability, as shown by the modifications of its movements 

 which result from contact with foreign bodies ; it is able to 

 respire ; it carries on processes of excretion ; and, finally, it is 

 capable of reproducing its kind. It is these functions that charac- 

 terise living beings as distinguished from non-living matter. 

 What is specially characteristic of the living organism in general 

 when compared with a non-living object is the capacity of the 

 former to respond by changes in itself to influences operating on 

 it from without. In the case of such an extremely simple 

 organism as Amoeba, these changes are also, necessarily, extremely 

 simple ; but they are of a quite definite character. In addition 

 to the effects produced on its actions by mechanical obstacles and 

 the presence of food-particles, it can be shown by experiment that 

 Amoeba responds by definite changes in itself to such external 

 influences as changes in the amount of oxygen supplied, in the 

 quantities of various salts present, in the temperature, and in the 

 electric conditions of the water in which it lives. The power of 

 locomotion, the capacity for assimilating organic substances, and 

 the absence of two special compounds chlorophyll and cellulose- 

 are specially characteristic of the animal as distinguished from 

 the plant. 



2. THE ANIMAL CELL. 



In all but the lowest animals the various functions just enume- 

 rated are carried on by means of a more or less complex machinery 



