io8 MICRO-ORGANISMS AND DISEASE [chap. 



to be noticed. When such a pellicle is broken up by 

 shaking, it in most instances falls to the bottom of the fluid, 

 and then after another day's growth a new pellicle appears, 

 and in this also copious spore formation is noticed ; and 

 this can be repeated for several days till the nutriment is 

 exhausted. The same can be seen in hay infusion, in the 

 case of bacillus subtilis. In neutral or faintly alkaline hay 

 infusion kept at 37° C. spores of bacillus subtilis are present 

 in the pellicle as early as the second day, and continue to 

 be formed till the end of eight to ten days. The view has 

 been expressed by some observers, amongst them Buchner, 

 that the spore formation in bacilli occurs on exhaustion of 

 the nutritive material, but it seems that the facts just men- 

 tioned as to the continuous and successive pellicle and spore 

 formation occurring in broth are incompatible with that 

 assertion ; and, besides, the formation of spores in other 

 bacilli can be shown to take place long before any exhaustion 

 of the nutritive matter is noticeable — e.g., in anthrax bacilli, 

 in the tetanus bacillus, and others. In the bacillus filamen- 

 tosus, growing on Agar or on potato, the spore formation is 

 apparent even before the first day is over and long before 

 the active growth and multiplication of bacilli all round 

 is finished. In some species, however, that do not thrive 

 under free access of air {e.g., oedema bacillus, tetanus ba- 

 cillus) spore formation does not take place if free oxygen is 

 present. A temperature of at least 16° C. is required for the 

 formation of spores, though spore formation occurs in all 

 temperatures between that and 45°; at least spore formation 

 has been seen to occur in bacillus anthracis even at 45° C. 

 The mode of spore formation hitherto described is called 

 that of endo-spores, and it ought to be here stated that many 

 species of baciUi exist in which no spore formation can be 

 demonstrated — in this statement we rely on the morpho- 



