VI] GENERAL CHARACTERS OF BACTERIA 107 



relatively few and far between. The last phase is reached 

 when the bacillus itself swells up into a gelatinous capsule 

 enveloping the spore and ultimately altogether disappears ; 

 then the spore is free and has reached its full size and 

 development. Examining in stained specimens spore- 

 bearing bacilli, the spores appear unstained, whereas the 

 rest of the bacillary substance takes readily the dye ; under 

 these conditions the spore looks like an oval clear space, 

 not unhke the vacuoles above mentioned ; but the spore 

 has a sharp outline of its own, the vacuole has not. It is, 

 however, not easy to distinguish in a given specimen, 

 stained after the ordinary methods, the spores which are not 

 stained from vacuoles, and in these cases other methods of 

 staining must be resorted to. 



In order, then, to decide whether or not spores are pre- 

 sent in a bacillary species, the morphological investigation, 

 fresh aspect, special methods of staining, drying, and heating 

 have to be resorted to. 



The spore formation is associated with supply of oxygen 

 in all bacteria that generally live well under access of air ; 

 in some species this is more pronounced than in others, for 

 in some species — e.g. bacillus anthracis, and bacillus fila- 

 mentosus, spores are only formed — ceteris paribus — if oxygen 

 has free access, and no spores are formed if there is no free 

 supply of oxygen — e.g., deep in the fluid ; while in other 

 species, though spore formation is greatly enhanced by the 

 free access of oxygen, it nevertheless takes place to a certain 

 limited extent deep in the fluids. Thus, in the case of 

 bacillus subtilis and various other bacilli a pellicle soon 

 makes its appearance on the surface of the fluid (broth, &c.). 

 This pellicle is made up of filaments and bacilli matted 

 together, and in them copious spore formation is going on, 

 but also in the depth there are a few spore-containing bacilli 



