10 



longer when inoculated into the subcutaneous tissues, where they 

 give rise to a localised abscess. 



The following is the record of an unsuccessful attempt to 

 demonstrate the production of rough forms of pneumococci in an 

 immunised mouse. A virulent Type I. culture in broth was 

 mixed with an equal quantity of a Type I. serum and incubated for 

 half an hour, after which the mixture was inoculated intra- 

 peritoneally into two mice. A small amount of peritoneal fluid 

 was withdrawn from each mouse one and two hours later, part of 

 which was sown on plates. Only smooth colonies of virulent type 

 grew, but mice inoculated with the rest of the fluids remained well. 

 The two original mice were killed 20 hours after inoculation and 

 the peritoneal cavities were washed out with salt solution. The 

 centrifuged deposits from the washings were sown on blood agar 

 plates ; no growth of pneumococci was obtained. 



If there is an intermediate stage in the mechanism of protec- 

 tion during which rough attenuated pneumococci, that is, cocci 

 which have not developed the part of their external structure 

 associated with the secretion of soluble substance, are produced 

 in a passively immune mouse, it is clear that they would readily be 

 destroyed, since even large doses of these R cocci inoculated into 

 an unprotected mouse quickly disappear. 



On the other hand, it may not be necessary to postulate such 

 an intermediate stage. The immune serum may protect the 

 mouse by depriving the pneumococcus of its virulent potentialities, 

 possibly by precipitating the external covering associated with 

 capsule formation and the secretion of soluble substances. The 

 pneumococci so affected would become the prey of the phagocytes. 

 They would, if removed from the animal before destruction, be 

 capable of multiplying, and as the exposure to the immune serum 

 had been brief, they would produce normal virulent descendants. 

 But if within the animal body some sensitised pneumococci 

 escaped the attacks of the leucocytes and divided, the descendants 

 would remain under the influence of the serum, in which case the 

 property of the serum to produce progressive attenuation might 

 be brought into action. 



Discussion of Results. 



1. The first point of interest which emerges from this study 

 of S and R forms of pneumococci is the fact that a definite 

 morphological criterion has been found to distinguish between 

 virulent and attenuated strains. Further evidence of attenuation 

 is furnished by serological reactions, in particular by the loose 

 character of the clumps in the agglutination test, and by the 

 absence of specific soluble precipitable substance. Given these 

 data, it will be possible to decide whether a culture is in the 

 condition suitable for the production of disease. 



2. An important practical corollary will be that, for the 

 production of good immune sera, strains should be used which 

 consist entirely of S forms. Immunisation of rabbits with the 



