inoculated mice a soluble substance which gave a precipitate 

 when the peritoneal washings, cleared by centrifugalisation, 

 were added to each of the three type sera. Absorption of I 

 and II sera with I and II strains removed the agglutinins and 

 bacteriotropins for all her nine strains, but absorption with one 

 of these nine strains removed these substances for itself only, 

 not for I or II strains nor for the other 8 strains. She prepared 

 immune sera with her strains and found that they were serolog- 

 ically distinct, though with some interrelationship between 

 certain of them ; the sera had no activity towards strains classed 

 as Type I or Type II or "atypical II." The suggestion of 

 underlying interrelationship between antigenically different strains 

 is also provided by the work of Laura Stryker,* who observed the 

 effect of prolonged culture in homologous serum. After this 

 treatment, Type I strains to a large extent lost their agglutinability 

 with Type I serum, and became agglutinable with Type II serum ; 

 Type II strains gave corresponding results, losing their agglutin- 

 ability by the homologous serum and becoming agglutinable by 

 Type I serum. These changes in serological reactions were 

 accompanied by loss of virulence and reduction of antigenic 

 capacity. 



It will, of course, be recognised that instances such as those 

 which I have quoted do not carry much weight and can only be 

 regarded as minor exceptions from the general rule. The main 

 fact which remains is that antigenic differences amongst pneumo- 

 cocci are closely reflected in the sharp differences between the 

 agglutinins produced by different strains. 



Precipitin Tests. 



The precipitin test, whilst affording indications that all 

 strains of pneumococci possess something in common, gives 

 results which correspond very closely with those obtained by 

 agglutination. Blake, f who has paid special attention to 

 differentiation of pneumococci by this test, gives the following 

 example : — 



The reactions were read after two hours at 37° C. and overnight on ice. 

 The antigen was prepared by drying in vacuo the washed pneumococci 

 from an 18-hour 1,000 c.c. broth culture; the dried material was taken up 

 in saline (10 mg. per c.c.) and repeatedly frozen and thawed until a faintly 

 opalescent fluid free from bacterial bodies was obtained. 



* Journ. Exper. Med., XXIV, p. 49. 1916. 

 t Journ. Exper. Med., XXVI, p. 67. 1917. 



A 4 



