From the preceding brief review of the relation of certain bacteria to some of the elements in the free state and from the further fact that there is scarcely a known natural organic compound which cannot be utilized by some kind of bacterium, it is evident that this class of organisms has a far wider range of adaptability than any other class, and this adaptability helps to explain their seemingly universal distribution. As to the metabolism _within the cell_, no more is known than is the case with other cells, nor even as much. The materials used for growth and as sources of energy are taken into the cell, built up into various compounds some of which have been enumerated and in part broken down again. Carbon dioxide and water are formed in the latter process. What other katabolic products occur it is not easy to determine. Certainly some of the substances mentioned in the next chapters are such products but it is not always possible to separate those formed _inside_ the cell from those formed _outside_. Perhaps most of the latter should be considered true metabolic products. It would seem that on account of the simplicity of structure of the bacterial cell and of the compounds which they may use as food they would serve as excellent objects for the study of the fundamental problems of cell metabolism. Their minuteness and the nearly impossible task of separating them completely from the medium in or on which they are grown makes the solution of these problems one of great difficulty. When all of the environmental conditions necessary for the best development of a given bacterium are fulfilled, it will then develop to the limit of its capacity. This development is characterized essentially by its reproduction, which occurs by transverse division. The rate of this division varies much with the kind even under good conditions. The most rapid rate so far observed is a division in eighteen minutes. A great many reproduce every half-hour and this may be taken as a good average rate. If such division could proceed without interruption, a little calculation will show that in about sixty-five hours a mass as large as the earth would be produced. Starting with 1 coccus, 1µ in diameter, its volume = 0.0000000000005 cc. 1/2 hour = 2 1 hour = 4 2 hours = 16 4 hours = 256 5 hours = 1024 = 10³+ 15 hours = 1,000,000,000 = 10⁹ = 0.5 cc. 35 hours = 10²¹+ = 500.0 cu.m. About 65 hours = 2 × 10⁴²+ = 5 × 10²⁰ cu.m. = a mass as large as the earth. Such a rate of increase evidently cannot be kept up long on account of many limiting factors, chief of which is the food supply.