SYSTEMATIC AND DESCRIPTIVE. 307 



double their length, and then undergo division. 

 They also form threads which may b6 composed 

 of long rods, short rods, and cocci. They are 

 motile, and provided with a flagellum at each end. 

 If the nourishing medium is impoverished, the 

 multiplication of the rods by division gradually 

 ceases, and spore-formation commences. The rods 

 become motionless, and a dark spot is visible, either 

 in the middle or towards one end. This gradually 

 develops into a shining spore with a dark contour. 

 The rods swell slightly during this process, their 

 contour becomes undefined, and soon disappears 

 entirely, so that the spores are set free in about 

 twenty-four hours. The spores are i'2 //> long, and 

 '6 p, broad. They develop into rods in the following 

 way. On one side of the spore a swelling appears, 

 at the summit of which an opening in the spore- 

 membrane results, and the germ escapes. This 

 lengthens into a rod, and remains for, a time 

 attached to the empty spore-membrane. These 

 spores are widely distributed, and occur in the air, 

 soil, dust, etc. On the excrement of herbivorous 

 animals the bacilli form a white efflorescence, and 

 on infusion of horse-dung a thick crumpled skin. 

 They flourish equally in liquids and upon damp, 

 solid, nourishing media. On potatoes they grow as 

 a yellowish-white skin ; on ordinary nutrient liquids 

 they develop a thin, and subsequently .a thick, 

 dense, crumpled pellicle, with copious spore-forma- 

 tion. They are aerobic ; deprivation of oxygen 



