ix.] BACILLUS. 71 



spore-formation has occurred ; and as has been stated above 

 the sheath swells up and remains for some time as a hyaline 

 gelatinous capsule around the spore, but sooner or later this is 

 also lost and the spore becomes quite free. When spore-form- 

 ation has taken place in a convolution or in a mass of lepto- 

 thrix, and after the sheaths of the bacilli have become swollen 

 up into a gelatinous matrix, it looks as if we had a zoogloea, 

 in which the bright oval spores form the particular elements 

 embedded in a more or less hyaline gelatinous matrix. But 

 even in these cases on careful analysis it is noticed that the 

 spores have a linear or serial arrangement, being originally 

 developed in filaments. 



This spore-formation occurs in many species of bacilli, and it 

 closes the cycle of the life-history of the bacilli But it does 

 not take place under all circumstances. In the case of many 

 bacilli, e.g. hay-bacillus, anthrax -bacillus, bacillus of putrefac- 

 tion, spore-formation occurs only when there is an ample 

 supply of oxygen, e.g. when the bacilli grow on the surface of 

 the nourishing material (Cohn, Koch). It has nothing what- 

 ever to do with the exhaustion of the nourishing material, as 

 Buchner seems to think ; for if the conditions of spore-form- 

 ation are given, amongst these particularly the exposure to 

 the air, bacilli will commence to form spores long before the 

 nourishing material is exhausted. I will here mention a 

 particular instance to show this. 



Take a test-tube with Agar-Agar peptone, such as has been 

 mentioned in a former chapter as fit for inoculation ; inoculate 

 the surface of the Agar-Agar with hay-bacillus or anthrax- 

 bacillus, place it in the incubator, and keep it there at a 

 temperature of 30 to 35 C. After 36 to 48 hours you will 

 find the surface covered with a good crop of bacilli and lepto- 

 thrix, and in some of them spore-formation is already going 

 on with great vigour. For several days after, the amount of 

 leptothrix increases, and in the filaments large numbers of 

 spores are formed. This goes on for several weeks, long before 

 the nourishing material becomes exhausted. But during all this 

 time the spore-formation is limited only to the surface ; the 

 filaments growing into the deeper strata remain without spore- 

 formation. The same observation can be made with gelatine 

 mixtures of peptone, broth, &c., in the test-tube or in the glass 

 cell described and figured in Chapter V. In the case of the 

 gelatine mixture it is particularly instructive to watch this 

 process, since it clearly proves that the free access .of air is 

 essential for the formation of spores. For if the anthrax- 

 bacillus be grown on the (solid) mixture of gelatine and broth 



