THE ORIGIN OF ANTITOXIN THE SIDE-CHAIN THEORY 97 



those which have been lost. Now suppose a fresh dose of toxin 

 reaches the cell, and that these new receptors are in their turn 

 taken up by toxin, but yet not in numbers sufficient to kill the 

 cell. The same processes will occur : the receptors will be rendered 

 useless, and a fresh crop of the same nature will be produced. 



Now in accordance with the well-known laws of habit, in virtue 

 of which a part of the body can gradually be trained to perform a 

 function which is difficult at first, but which becomes more easy 

 by usage, the molecule of protoplasm gradually acquires the 

 faculty of producing these receptors more and more easily, in 

 obedience to nicely graduated doses of toxin. If these are 

 repeated in proper amount and at suitable intervals the cell may 

 be trained, so to speak, to produce these receptors, and it may 



FIG. 23. 



ultimately come to do so in excess and far beyond its physiologi- 

 cal requirements. This also is analogous with known biological 

 facts, such as the production of callus in a fractured bone in 

 excess of the amount necessary for repair. We shall revert to 

 this point later. Now we have to imagine that these receptors 

 may be formed in excess so great that they cannot all remain 

 attached to the cell ; some of them are pushed off, so to speak, by 

 the younger ones, which are growing up to take their place. If 

 this occurs, these receptors will pass off freely into the blood. As 

 we have seen, they are so constituted that they will unite speci- 

 fically with the particular toxin that was injected. These cast-off 

 receptors constitute antitoxin, and the formation of that substance is 

 simply due to the pathological separation of the groups of atoms, 

 in virtue of which the toxin molecule is linked up to the living 



7 



