THE UNICELLULAR BODY. 157 



Nevertheless the one-celled organism perforins all of the 

 characteristic operations of life. A single mass of protoplasm, a 

 single cell, unites in itself the performance of all the various 

 elementary functions which in the multicellular forms are distrib- 

 uted among many cells, differentiated into divers tissues and 

 organs. The unicellular forms are therefore in a physiological 

 sense as truly ' ' organisms ' ' as the multicellular forms ; and in 

 many cases the unicellular body shows a very considerable degree 

 of differentiation among its parts. But the unicellular forms 

 are organisms reduced to their lowest terms ; they present us with 

 the problems of life in their most rudimentary form. Hence 

 they may afford a kind of key to the more elaborate organization 

 of the higher types. 



We shall find among unicellular forms representatives both 

 of animals and of plants, and to a detailed examination of some 

 of these we may now proceed. 



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