STRUCTURE, MOTIONS, REPRODUCTION. 121 
Koch in his classical study 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 microérganism 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 
Fie. 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. 
