MORPHOLOGY AND PHYSIOLOGY OF BACTERIA. 19 



plasm becomes granular, and at certain points in the thread a speck 

 appears, which gradually enlarges and develops into a circular 

 or egg-shaped, sharply defined, highly refractive body. The spore 

 grows at the expense of the protopJasm of the cell, which in time, 

 together with the cell-wall, entirely disappears, and the spore is set 

 free. These phenomena are best seen in an immotile bacillus in a 

 ■drop-cultivation on a warm stage ; the whole process may then be 

 observed continuously from beginning to end. Spores may form in 

 each link of the thread, so that a regular row results, or they may 

 occur at irregular intervals. Spore-formation also occurs in bacilli 

 which do not develop into leptothrix filaments. The spores may 

 develop in the centre or at one end of the rod. In the tetanus 

 bacillus a spore develops at the extreme end, producing the appear- 

 ance of a drum-stick. The spore may be considerably wider, but 

 is never longer than the parent cell. 



Fig. 6. — Lboconostoo mesenteboidbs ; Cocoi-ohains with Akthkospokes 

 (after Van Tieghem and Oienkowski). 



Arthrospore formation is illustrated in Leuconostoc mesenteroides. 

 ■Certain elements in the chain of cocci, apparently not differing from 

 the rest, become larger, with tougher walls, and more refractive. 

 The remaining cells die, and these cells having acquired the pro- 

 perties of spores are set free, and can reproduce a new growth in 

 any fresh nourishing soU. That this occurs in all species which 

 do not form endospores is at present only a supposition. 



Spores are invested by a thick membrane, which is beHeved to 

 consist of two layers. To this they probably owe the property they 

 possess of retaining vitality when desiccated, and of offering a, 

 greater resistance to the action of chemical reagents and heat than 

 the parent cells. > 



Spore-formation has been regarded by some as occurring when 

 the nourishing soil is exhausted, thus providing for the perpetuation 



