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disintegrate, with the liberation of wore "lytic substance"; 

 so the process is repeated, the net result being rapid " growth " 

 of "lytic substance." At the same time those of the variants 

 which are viable subdivide into a varying proportion of viable 

 and non- viable forms, from the latter of which the supply of 

 " lytic substance " is again increased. 



Experiments with lytic substances raise interesting, and often 

 puzzling, questions as to the interplay between specific and non- 

 specific factors in the production of variants. 



Here are some instances. A lytic substance derived from 

 a particular strain may (1) act on all strains of the same species, 

 or (2) it may fail to act on some of the strains. Is (1) due to 

 identity, and (2) to lack of identity in the receptor apparatus ? 

 That would seem the simplest explanation. Further, when 

 the lytic substance acts on some other species, as well as on the 

 species from which it was derived, is that due to community of 

 " group " receptors ? This may be the case, but it is not so 

 easily acceptable. Here " group " receptors, in the purely 

 chemical sense of the term, may have to give way to more physical 

 conceptions; for example, the colloidal condition of bacterial 

 protoplasm may differ in different bacterial species and may be 

 influenced in some cases, but not in others, by the finely-divided 

 colloids in the filtrate containing a particular lytic principle. 



Still, it is worth considering how far it may be possible to 

 apply the " receptor theory." Different stimuli may be equally 

 effective in causing a culture to produce lytic substance. For 

 example, a stimulus, a, may cause a normal culture of bacillus C 

 to produce lytic substance c 1 , which may be transmitted to a 

 fresh normal culture, and so on, indefinitely. Another stimulus, 

 6, may produce the same effect on bacillus C, yielding a supply 

 of lytic substance c 2 , which may be propagated ad infinitum. 

 One might suppose that, after several transmissions through 

 identical cultures of normal bacillus C, c 1 and c 2 would be com- 

 pletely identical with each other. But this may not be the case. 

 If stimulus a was a filtrate from a culture of bacillus A (an organism 

 different from C), whilst stimulus b was derived from some other 

 source, and if the lytic properties of the propagated c 1 and c 2 

 be tested on a culture of A, it may be found that c 1 produces 

 lysis, and that c 2 fails to do so. How does c 1 " remember " 

 its affinity for A ? Did a cause the protoplasm of C to break 

 up in a special way, viz., into particles containing receptors 

 for both C and A ? And was this principle perpetuated in the 

 transmission of c 1 from culture to culture of C ? 



This idea of linking up of two (or more) kinds of receptors 

 leads to a further question. By immunisation with bacterial 

 filtrates containing a lytic principle, " antilytic " sera, i.e., sera 

 which neutralise or inhibit lytic action, may be produced. Some 

 of these neutralisation experiments suggest that one may be 

 dealing with a combination of receptors (A and B), of which one 

 (A) is dominant and the other (B) is in abeyance or " masked." 



