122 STRUCTURE, MOTIONS, REPRODUCTION. 



75 c) ; or one extremity may be enlarged, producing forms such as 

 are shown in Fig. 75, d. Some of the smaller spherical spores meas- 

 ure less than 0. 5 //. in diameter, but they are, for the most part, oval 

 bodies having a short diameter of 0.5 to 1 //- and a long diameter of 

 one to two /^, or even more. They are enveloped in a cellular en- 

 velope which, according to some observers, consists of two layers 

 an exosporium and an endosporium. 



The mode of spore formation shown in Fig. 75, c and d, has been 

 adopted by some authors as a generic character. When the spores 

 are located in the central part of the rods, giving rise to a spindle- 

 shaped body, as at c, the bacilli are assigned to the genus Clos- 

 tridium; when located at one end, as at d, the bacilli are shaped 

 like a drumstick, and this mode of spore formation is used as the 

 distinguishing characteristic of the genus Plectridium. Hueppe 

 groups all rod-shaped bacteria which form endospores under the 

 generic name Bacillus, with three sub-genera : Bacillus, straight 

 rods; Clostridium, spindle-shaped rods; Plectridium, drumstick- 

 shaped rods. 



The germination of spores has been studied by Prazmowski, 

 Brefeld, and others. The process is as follows : By the absorption 

 of water they become swollen and pale, losing their shining, refrac- 

 tive appearance. Later a little protuberance is seen upon one side 

 or at one extremity of the spore, and this rapidly grows out to form 

 a rod which consists of soft-growing protoplasm enveloped in a 

 membrane which is formed of the endosporium or inner layer of the 

 cellular envelope of the spore. The outer envelope, or exosporium, 

 is cast off and may be seen in the vicinity of the newly formed rod 

 (Fig. 76). Sometimes the vegetative cell emerges from one extrem- 



ity of the oval spore, as shown at a, Fig. 76, and in other species the 

 exosporium is ruptured and the bacillus emerges from the side, as 

 seen at b. 



The considerable resistance of these endogenous spores to desic- 

 cation, to heat, and to various chemical agents is an important fact 

 both from a biological and from a hygienic point of view, and will 

 be fully considered in a subsequent chapter. The fact that certain 

 bacilli and spirilla do not withstand a temperature of 80 to 90 C., 

 which does not destroy the vitality of known spores, leads to the in- 



