118 THE SIDE-CHAIN THEORY 



processes must take place) are chemically alike, and that certain side 

 chains are peculiar to a certain cell type, while others are common 

 to all cells. This conception of the chemical structure of the cell 

 may be diagrammatically expressed by representing the functional 

 centre as a sphere, from which variously colored rays the side 

 chains, emanate; and the difference between a nerve cell, for example, 

 and a connective-tissue cell or muscle cell could be expressed by the 

 presence of a different percentage of rays of a common color and the 

 additional presence of special rays of differing tints. This I have 

 attempted to illustrate in the accompanying illustration (Plate I). 



It will be seen that all three types of cells have a, b, c, d, and e 

 rays in common, but that these are present in different percentage 

 proportions, and that the cells differ from one another riot only in 

 this respect, but also in the exclusive presence of / rays in the one, 

 of g rays in the other, and of h rays in the third. Ehrlich further 

 conceives that a given foodstuff can only be utilized by a given cell, 

 if it possesses atomic groups which are capable of combining with 

 corresponding groups of the side chains. As the latter are not all 

 alike in their chemical structure, it is reasonable to suppose that 

 a definite relationship must exist between their combining groups 

 and the combining groups of the foodstuffs, such that certain food 

 molecules only will be capable of uniting with certain side chains. 



To use the frequently quoted simile which Emil Fischer first 

 applied to the specific action of ferments, we may say that the 

 combining group of the food molecule must fit a corresponding group 

 of the side chain like a complex key fits its special lock. To revert 

 to our diagram we may express this by assuming that only a black 

 food molecule can combine with a black side chain, only a green 

 one with one of its own color, etc. 



All those side chains which are capable of combining with chemical 

 bodies in general, Ehrlich designates collectively as receptors, or 

 chemoreceptors, while those which react with foodstuffs more or less 

 exclusively, and which accordingly serve the nutrition of the cell, 

 are appropriately termed nutriceptors. 



Under ordinary conditions of cell life, we can well imagine that 

 the cell receptors will have occasion to react only with actual food- 

 stuffs. But we can also conceive that under abnormal conditions, as 

 in the various infections, substances may be brought to the cell which 

 accidentally possess an atomic group that is identical in structure 



