THE PROBLEM 47 



zoan cell, and the whole metasocm soma, not one of its 

 constituent cells. In the protozoan, all the diiferentia- 

 tions are in and a part of one single cell operating as one 

 metabolic unit, of small absolute size, and consequently- 

 easier and more labile internal physico-chemical regula- 

 tion. In the metazoan soma we have organ differentia- 

 tion, with the constituent cells in each organ highly 

 specialized functioiially, and dependent upon the nor- 

 mal functional activity of wholly other organs in order 

 that they may keep going at all. Remove these 

 tissue cells from the soma, and provide them with an 

 abundance of suitable nourishment and oxygen, as in 

 tissue cultures, and, so far as the evidence now available 

 indicates, they will live forever (cf. Chapter II). 



Consider for a moment the most highly differentiated 

 protozoan known, on the one hand, and man, on the other 

 hand, purely as physico-chemical machines, which only 

 keep going if the internal balances and adjustments are, 

 in each case, held within a narrow zone of normality. 

 Quite aside from any question of their different modes 

 of reproduction, the two machines are not equivalent, 

 as machines, because of :(a) unicellular versus multicel- 

 lular structure, (b) great absolute difference in size of 

 the whole machines, with consequent requirement of an 

 enormously more complex internal regulatory mechanism 

 in the one case than in the other, whatever the inherent 

 nature of this mechanism may be. 



Essentially the same view of the matter as that 

 held by the present writer has been well set forth by Loeb 

 in his most recent paper on the subject. He says : 



"All this points to the idea that death is not inherent in the individual 

 cell, hut is only the fate of more complicated organisms in which different 

 types of cells or tissues are dependent upon each other. In this case it 

 seems to happen that one or certain types of cells produce a substance or 

 substances which gradually become harmful to a vital organ like the res- 



