12 PHYSIOLOGY CHAP. 



In the Protista each cell is a distinct and independent physio- 

 logical individual ; in the Metazoa each cell or cell-derivative is 

 still a distinct individual, but it is no longer independent, since 

 the life of each is more or less bound up with the life of the others 

 with which it is associated. The individuality of the social aggre- 

 gate, or that of the organism as a whole, is but an individuality 

 of a higher order, i.e. it is the sum of the life of each elementary 

 organism. This is essentially the Cell Theory, formulated by 

 Schleiden (1838) and Schwann (1839), reinforced and developed 

 by Virchow (1855), and fully confirmed by later observers. 



Yet among living physiologists there are not wanting some 

 who believe that we must recognise a more radical difference 

 between the independent unicellular organisms and the cells of 

 which complex organisms are built up. The latter, it is said, since 

 they are incapable of living apart from the body of which they 

 form a part, do not constitute a real individual, BO that the name 

 of elementary organisms given them by Briicke is inappropriate. 

 Since the several physiological functions essential to life are very 

 unequally divided among the various cells of which the complex 

 organism consists, they must each represent a physiologically 

 simpler unit, and are not therefore comparable with the cells that 

 constitute a true individual, and which are capable ' of living 

 independent of other cells (E. Schenk and J. Loeb). 



There is a certain amount of truth in this observation, but the 

 conclusions deduced from it, i.e. the negation of the cell theory, 

 are somewhat far-fetched. In the first place it should be noted 

 that incapacity to live independent of other cells cannot be predi- 

 cated of all the cells of which multicellular organisms are composed ; 

 it rises gradually with the zoological scale (cf. Chap. III. 12). It 

 should further be observed that the life of every organism is 

 invariably conditioned by its special environment, so that it perishes 

 when transported into other media too unlike those in which it 

 normally exists. In unicellular organisms the environment is 

 represented by the sum of the nutritive materials and the stimuli 

 which reach them from the external world ; in the cells of which 

 multicellular organisms are built up the medium is represented 

 by the sum of the nutritive matters and the stimuli which reach 

 them, either from the external world or from the other cells with 

 which they live in association. Lastly, in the first as in the 

 'second kind of cell a different grade or trend of development may 

 be observed for each of their vital functions. 



For the rest, the cell theory, which affirms a certain functional 

 autonomy of the morphological elements of which the organism as 

 a whole consists, is founded on a synthesis of experimental facts 

 that can be easily verified, 



(a) The survival for a certain time of parts detached from a 

 living organism. 



