Life-history of the Meningocoeetis. 367 



employed, to invoke the theories of involution forms, or of mutation, 

 in order to explain these genuine types of pleomorphic activity. 

 A more reasonable, and much simpler, explanation of bacterial 

 pleomorphism lies, in short, in the view that these aberrant types 

 merely represent different stages in the life-history of bacteria. 

 And this view i^ strongly supported, if not absolutely proved, by 

 actual observation, on the warm-stage, of the development of single 

 individuals. And in this connexion it is well to remember 

 that excellent precedent is afforded for this explanation by de- 

 monstration of the fact that the Discomyces and Sporotricha, to 

 mention only two groups, are not bacterial at all, as they were for 

 long considered to be. 



So much by way of inti'oduction. 



To-day Mr. Martin Duncan and I propose to put before you 

 fresh evidence in support of my thesis that the lower bacteria 

 are not necessarily what they seem to be. And the organism 

 I have selected for this purpose is the meningococcus of Weichsel- 

 baum. 



It is generally taught, and it is widely believed, that the 

 meningococcus of Weichselbaum is the direct casual agent of that 

 form of epidemic cerebrospinal fever with which the meningococcus 

 is usually associated. 



It is also taught, and it is generally accepted, that the 

 meningococcus — as one of the "lower" bacteria — is an organism 

 with a simple life-history, reproduction taking place exclusively by 

 the process of binary fission into two equal parts, a meningococcus 

 always arising from a meningococcus, and in no other way. 



As regards the first of these jDropositions, that the meningo- 

 coccus is itself the cause of epidemic cerebrospinal fever, I showed 

 by experiment in 1915 — and this, in conjunction with Captain 

 Caulfeild, I amply confirmed in the following year — that the theory 

 cannot be accepted for this simple reason, viz. that cultures of the 

 meningococcus in cerebrospinal fluid, or in laboratory media, 

 frequently contain a filterable virus which is just as pathogenic to 

 monkeys (though not necessarily productive of the disease) as are 

 unfiltered cultures. 



It follows from this, notwithstanding many statements to the 

 contrary, that the disease has not yet been produced experimentally 

 in animals by injection of pure cultures of Weichselbaum's organism. 

 There is, in fact, no evidence that the meningococcus as such is 

 responsible for the disease. 



It is true that an attempt was made to discredit my results by 

 the suggestion that the filterable virus referred to was a toxin. 

 But there is no toxin known to us which is directly capable of pro- 

 ducing continued fever lasting for many weeks, after the lapse of 

 a well-marked latent period. And completely negative bacterio- 

 logical examinations of the animals in question excluded the 



