VEGETATIVE REPRODUCTION takes place by continued transverse division; hence the name “Fission-Fungi” or “Fission-Algæ,” has been applied to the Bacteria. SPORES. The spores are probably developed in two ways. In the ENDOSPOROUS species (Figs. 28, 29), the spore arises as a new cell inside the mother-cell. The spores are strongly refractile, smaller than the mother-cell, and may be compared to the aplanospores of other Algæ. In addition to these there are the ARTHROSPOROUS species in which the cells, just as in _Nostoc_ and other Blue-green Algæ, assume the properties of spores without previously undergoing an endogenous new construction, and are able to germinate and form new vegetative generations (Fig. 27). The formation of spores very often commences when the vegetative development begins to be restricted. [Illustration: FIG. 27.--_Leuconostoc mesenterioides_: _a_ a zooglœa, natural size; _b_ cross section of zooglœa; _c_ filaments with spores; _d_ mature spores; _e-i_ successive stages of germination; in _e_ portions of the ruptured spore-wall are seen on the external side of the mucilaginous covering. (_b-i_ magnified 520.)] The spores germinate as in _Nostoc_ by the bursting of the external layer of the cell-wall, either by a transverse or longitudinal cleft, but always in the same way, in the same species (Fig. 28, example of transverse cleft). DISTRIBUTION. Bacteria and their germs capable of development, are found everywhere, in the air (dust), in surface water, and in the superficial layers of the soil. The number varies very much in accordance with the nature of the place, season, etc. They enter, together with air and food, into healthy animals and occur always in their alimentary tract. GROWTH AND REPRODUCTION depend upon the conditions of temperature. There is a certain minimum, optimum and maximum for each species; for instance (in degrees Centigrade)-- Minim. Opt. Maxim. _Bacillus subtilis_ + 6 c. 30 + 50 _B. anthracis_ 15 20–25 43 _Spirillum choleræ asiaticæ_ 8 37 40 (but grows only feebly if under 16°). _Bacterium tuberculosis_ 28 37–38 42