STRUCTURE, MOTIONS, REPRODUCTION. 121 



Koch in his classical stud}' of the anthrax bacillus (1878), and by 

 Cohn, who studied the formation of spores in Bacillus subtilis. 



These reproductive bodies serve the same purpose in the preserva- 

 tion of species as the seeds of higher plants. They resist desiccation 

 and may retain their vitality for months or years until circumstances 

 are favorable to their development, when, under the influence of heat 

 and moisture, they reproduce the vegetative form bacillus or spiril- 

 lum with all of its biological and morphological characters. They 

 are composed of condensed protoplasm which retains the vital char- 

 acters of the soft protoplasm of the mother cell from which it has 

 been separated ; and it is evident that whether reproduction occurs 

 by fission or by the formation of endogenous spores, the protoplasm 

 of the cells in a pure culture of any microorganism is simply a sepa- 

 rated portion of the protoplasm of the progenitors of these cells. 



Some of the bacilli grow out into long filaments before the forma- 

 tion of spores occurs ; and these filaments may be associated in bun- 

 dles or intertwined in irregular masses. At first the protoplasm of the 



FIG. 75. 



filaments is homogeneous, but after a time it becomes segmented, 

 and later the protoplasm of each segment becomes condensed into 

 a spherical or oval refractive body, which is the spore. For a time 

 these are retained in a linear position by the cell membrane of the 

 filament (Fig. 75, a), but this is after a while dissolved or broken 

 up and the spores are set free. In liquid cultures they sink to the 

 bottom as a pulverulent precipitate, and upon the surface of a solid 

 medium they form a layer which is usually of a white or yellowish- 

 white color, and which, when examined under the microscope, in old 

 cultures is found to consist almost entirely of shining spherical or 

 oval bodies which do not stain, by the ordinary methods, with the 

 aniline colors. While many of the bacilli during the stage of spore 

 formation grow out into long filaments, others do not, and one or 

 more spores make their appearance in rods of the ordinary length 

 which characterizes the species. These may be located in the centre 

 of the rod or at one extremity (Fig. 75, b). It sometimes occurs 

 that when a single central spore is formed the rod becomes very 

 much enlarged in its central portion, assuming a spindle shape (Fig. 



