4 8o SCIENCE PROGRESS 



So that if we were to take normal behaviour as the basis of 

 comparison between different cells we should soon be flounder- 

 ing in a hopeless confusion of structural and functional characters. 

 And even if the particular case which we might consider were 

 one into which structural considerations did not enter, still the 

 behaviour of any cell examined would probably be only in part 

 the outcome of the cell's own properties, and for the rest would 

 depend on the properties of other cells contained in the organism. 

 It would in fact be impossible to proceed on this basis with any 

 sort of certainty to compare two types of cell in the matter of 

 their individual functions. 



Therefore we need to form a conception of the functional 

 capability of a cell as distinguished from its normal behaviour. 

 Two cells must be considered to differ in functional capability 

 only if they react differently from like stimuli when placed under 

 identical conditions of environment. A concrete example may 

 make this point clearer. The cells which compose the heart- 

 muscle of a frog contract during life with a regular rhythm, 

 relaxation following contraction and contraction relaxation at 

 fixed intervals of time. The cardiac muscle cell of the lobster 

 behaves in a manner almost identical. The normal behaviour 

 of these two cells is therefore similar. But if we place the two 

 under identical conditions and subject them to like stimuli it 

 becomes clear that they differ considerably in functional capa- 

 bility. When subjected to a series of rapidly repeated electric 

 stimuli the cardiac muscle cell of the frog exhibits a refractory 

 phase, immediately succeeding each contraction, during which 

 the stimuli are unable to evoke any further contraction whatever. 

 The cardiac muscle cell of the lobster, however, remains in 

 complete contraction as long as the series of stimuli continues ; 

 it presents no refractory phase. This is an example of cells alike 

 in behaviour but different in functional capability. Similarly 

 one can find examples of the converse — difference of behaviour 

 with likeness of capability. Behaviour is in fact so little coinci- 

 dent with functional capability, that two cells may possess a 

 difference of capability which is completely latent, never showing 

 itself in the normal life of the cell as a difference of behaviour. 

 We must insist therefore that the basis of functional comparison 

 between cell and cell shall be the pure functional capability of 

 each, quite abstracted from the similarity or dissimilarity of 

 their normal behaviour under the possible influence of other 



