The motion of bacteria is characteristic and a little practice in observing will enable the student to recognize it and distinguish between motility and “Brownian” or molecular motion. Dead and non-motile bacteria show the latter. In fact, any finely divided particles suspended in a liquid which is not too viscous and in which the particles are not soluble show Brownian motion or “pedesis.” This latter is a dancing motion of the particle within a very small area and without change of place, while motile bacteria move from place to place or even out of the field of the microscope with greater or less speed. There is a marked difference in the character of the motion of different kinds of bacteria. Some rotate around the long axis when moving, others vibrate from side to side. Among the higher thread bacteria there are some which show motility without possessing flagella. Just how they move is little understood. =Spores.=--Under certain conditions some bacterial cells undergo transformations which result in the formation of so-called _spores_. If the process is followed under the microscope, the changes observed are approximately these: A very minute point appears in the protoplasm which seems to act somewhat like the centrosome of higher cells as a “center of attraction” so that the protoplasm gradually collects around it. The spot disappears or is enclosed in the collected protoplasm. This has evidently become denser as it is more highly refractive than before. In time all or nearly all of the protoplasm is collected. A new cell wall is developed around it which is thicker than the cell wall of the bacterium. This thickened cell wall is called the “spore capsule.” Gradually the remnants of the former cell contents and the old cell wall disappear or dissolve and the spore becomes “free” (Fig. 25). [Illustration: FIG. 25.--The smaller oval bodies in the middle of the field are free spores.] If the spore is placed in favorable conditions the protoplasm absorbs water, swells, the capsule bursts at some point, a cell wall is formed and the bacterium grows to normal size and divides, that is, it is an active growing cell again. This process is called “germination” of the spore. The point at which the spore capsule bursts to permit the new cell to emerge is characteristic for each kind of bacterium. It may be at the end when the germination is said to be _polar_ (Fig. 26). It may be from the middle of one side which gives _equatorial_ germination (Fig. 27). Rarely it is diagonally from a point between the equator and the pole, which type may be styled _oblique_ germination. In one or two instances the entire spore swells up, lengthens and becomes a rod without any special germination unless this type might be designated _bi-polar_.