INTRODUCTORY. 3 



prelimiDary changes in the raw food introduced into the body of 

 the amoeba ; and hence are retained within the body for some little 

 time. Such products are generally spoken of as ' secretions.' 

 Others which pass more rapidly away are generally called 'ex- 

 cretions.' The distinction between the two is an unimportant and 

 frequently accidental one. 



The energy expended in the movements of the amoeba is 

 supplied by the chemical changes going on in the protoplasm, by 

 the breaking up of bodies possessing much latent energy into 

 bodies possessing less. Thus the metabolic changes which the 

 food (as distinguished from the undigested stuff mechanically 

 lodged for a while in the body) undergoes in passing through the 

 protoplasm of the amoeba are of three classes : those preparatory 

 to and culminating in the conversion of the food into protoplasm, 

 those concerned in the discharge of energy, and those tending to 

 economise the immediate products of the second class of changes 

 by rendering them more or less useful in carrying out the first. 



5. It is respiratory. Taken as a whole, the metabolic changes 

 are pre-eminently processes of oxidation. One article of food, i.e. 

 one substance taken into the body, viz. oxygen, stands apart from 

 all the rest, and one product of metabolism peculiarly associated 

 with oxidation, viz. carbonic acid, stands also somewhat apart from 

 all the rest. Hence the assumption of oxygen and the excretion 

 of carbonic acid, together with such of the metabolic processes as 

 are more especially oxidative, are frequently spoken of together as 

 constituting the respiratory processes. 



6. It is reproductive. The individual amoeba represents a 

 unit. This unit, after a longer or shorter life, having increased in 

 size by the addition of new protoplasm in excess of that which it 

 is continually using up, may, by fission t (or by other means) resolve 

 itself into two (or more) parts, each of " which is capable of living 

 as a fresh unit or individual. 



Such are the fundamental vital qualities of the protoplasm of 

 an amoeba; all the facts of the life of an amoeba are manifesta- 

 tions of these protoplasmic qualities in varied seouence and sub- 

 ordination. 



The higher animals, we learn from morphological studies, may 

 be regarded as groups of amoebae peculiarly associated together. 

 All the physiological phenomena of the higher animals are simi- 

 larly the results of these fundamental qualities of protoplasm 

 peculiarly associated together. The dominant principle of this 

 association is the physiological division of labour corresponding to 

 the morphological differentiation of structure. Were a larger or 

 ' higher' animal to consist simply of a colony of undifferentiated 

 amoebae, one animal differing from another merely in the number 

 of units making up the mass of its body, without any differences 

 between the individual units, progress of function would be an 



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