Life-history of the MeningococcuF. 371 



re^^ards sterility by complete bacteriological and experimental tests, 

 monkeys being employed for the latter. 



By taking these precautions, and by replating at each stage and 

 retesting from single colonies, both with fermentation and sero- 

 logical tests, as absolute a guarantee as it is possible to obtain by 

 observation of cultures from single colonies is assured. 



However useful for purposes of identification, and for ensuring 

 sterilization as far as may be, cultivation from single colonies can- 

 not, nevertheless, give an absolute guarantee of sterility. To obtain 

 this only two methods are open to us. The first is to pick off 

 single meningococci from a solid culture, and to place each in- 

 dividual into a separate tube containing a suitable liquid medium, 

 in the hope of successful cultivation therefrom. 



This is the classical method, and in theory it is ideal. In 

 practice, however, it cannot be relied on, and for this reason. 

 As I have previously shown, the presence of the minute filterable 

 virus frequently to be found, and even to be seen as very minute 

 organisms, in meningococcal cultures, makes it impossible to be 

 certain that in picking up a normal-sized meningococcus one is not 

 at the same time picking up one of these smaller forms. Now, the 

 smallest of these forms are, as I have repeatedly tested, quite 

 invisible under a dry lens, and the highest compensating ocular that 

 will allow of good illumination. Even with a magnification of 

 3000, as obtained with a 1 • 5 mm. apochromatic oil-immersion lens 

 and the appropriate tube-length and compensating ocular, it is 

 still impossible to be certain that one organism, and one only, has 

 been picked up. The photograph of the camera lucida drawings 

 Mali illustrate this point, and this applies both to Barber's method 

 and to the fragmented cover-slip method. The only alternative, 

 therefore, is actually to watch on the warm-stage growth from a 

 single organism, and even here, on account of these extremely 

 minute forms, the problem is one of great difficulty. 



After innumerable trials I have however partially succeeded, 

 as you will see. 



Owing, however, to the fact that these fjiant forms are 

 apparently loth to grow on solid media, it became necessary to 

 devise a method of observing growth in liquid cultures, the hanging- 

 drop method of course being useless for the purpose owing to the 

 shape and depth of the drop. 



There are two ways, both of \\-liich you will see illustrated on 

 the screen, of studying growth in liquid cultures on the warm 

 stage. 



The first — not very satisfactory, owing to rapid drying — is to 

 make a thin film of serum-broth culture under a cover-slip on an 

 ordinary sterile slide covered with a thin layer of fresh serum-agar. 

 In successful cases growth of the giant forms, showing exogenous 

 gemmation and endosporulation, can be observed in the thin layer 



