This resistance explains why it happens that food materials boiled and sealed in cans to prevent the entrance of organisms sometimes spoil. The spores have not been killed by the boiling. It explains also in part the persistence of some diseases like anthrax and black leg in pastures for years. From the above description it follows that the spore is to be considered as _a condensation of the bacterial protoplasm surrounded by an especially thick cell wall_. _Its function is the preservation of the organism under adverse conditions._ It corresponds most closely to the encystment of certain protozoa--the ameba for example. Possibly the spore represents a very rudimentary beginning of a reproductive function such as is gradually evolved in the higher thread bacteria, the fission yeasts, the yeasts, the molds, etc. Its characteristics are so markedly different, however, that the function of preservation is certainly the main one. It must not be supposed that spores are formed under adverse conditions only, because bacteria showing vigorous growth frequently form spores rapidly. Special conditions are necessary for their formation just as they are for the growth and other functions of bacteria (Chapters VI and VII). CHAPTER III. CELL FORMS. Though there is apparently a wide variation in the shapes of different bacterial cells, these may all be reduced to _three_ typical _cell forms_. These are: first and simplest, the round or _spherical_, typified by a ball and called the _coccus_ form, or _coccus_, plural cocci[4] (Fig. 33). The coccus may be large, that is, from 1.5µ to 2µ in diameter. The term _macrococcus_ is sometimes applied to these large cocci. If the _coccus_ is less than 1µ in diameter, it is sometimes spoken of as a _micrococcus_; in fact, this term is very commonly applied to any coccus. When cocci are growing together, many of the cells do not appear as true spheres but are more or less distorted from pressure of their neighbors or from failure to grow to full size after recent division. Most cocci divide into hemispheres and then each half grows to full size. A few cocci elongate before division and then appear oval or elliptical. The second cell form is that of a _cylinder_ or rod typified by a section of a lead-pencil. The name _bacillus_, plural _bacilli_, is applied to this type (Fig. 34). The bacillus may be short (Fig. 35), 1µ or less in length, or long, up to 40µ in rare cases. Most bacilli are from 2µ to 5µ or 6µ long. The ends of the rod are usually rounded, occasionally square and very rarely pointed. It is evident that a very short rod with rounded ends approaches a coccus in form and it is not always easy to differentiate in such cases. Most bacilli are straight, but some are slightly curved (Fig. 36).