ska (Acta Soc. Bot. Poloniae, 7, 1930, 507), Beebe (Jour. Bact., U, 1941, 214) and Kliene- berger-Nobel (Jour. Gen. Microbiol., 1, 1947, 33). The myxobacterial colony, also sometimes designated as a swarm or pseudoplasmodium, consists characteristically of a flat, thin mass of vegetative cells which spreads rapidly over the surface of the substrate as a result of active movement from the periphery. Frequently, but not invariably, the cells lie on a thin membrane of slime. Colonies of a more compact nature may be produced if movement is impeded by copious slime formation or unfavorable nutrient conditions or if growth occurs on a gel of soft enough consistency (e.g., 1 per cent agar) to allow penetration of the cells into the substrate. In fruiting species, the fruiting bodies are formed characteristically in the older, central portion of the colony, often in a series of successive, concentric rings. Resting cells and fruiting bodies. Each resting cell is produced from a single, entire, vegetative cell. In the families Archangiaceae, Sorangiaceae and Polyangiaceae there is little structural change: the vegetative cell merely becomes somewhat shorter and thicker. In the famil}^ Myxococcaceae, the vegetative cell becomes converted to a mature resting cell which is spherical or ellipsoidal and which is surrounded by a refractile, deeply staining wall; these structures are commonly referred to as microcj'sts. In some genera the resting cells are borne in cysts, which enclose several hundred individual resting cells within a common membrane. Studies on the survival of the resting cells are still fragmentary. They do not appear to possess markedly greater thermal resistance than the vegetative cells but can survive desiccation for many years. Jahn reports germination of Polyangium fuscum after almost 6 years and of Myxococcus fulvus after 8 years. Except in the genus Sporocytophaga and the amicrocystogenous family Cytuphaguceae, the resting cells are found in structures known as fruiting bodies, each formed bj' the aggrega- tion and transformation of a large number of vegetative cells. Whereas the properties of the vegetative cells vary little from group to group, fruiting bodies differ widely in shape, size, structure and color; hence differentiation of families, genera and species is based largely on this character. Unfortunately, there have been relatively few studies on the range of variation of the fruiting structures under varying conditions of cultivation, so that the validity of the differences which have been employed for taxonomic purposes is difficult to assess. Ecology. Many species of fruiting myxobacters have been described as occurring on the dung of herbivores. The work of the Krzemieniewskis (Acta Soc. Bot. Poloniae, 5, 1927), Mishustin (Mikrobiologia, 7, 1938, 427), Singh (Jour. Gen. Microbiol., 1, 1947, 1) and others has shown, however, that they occur in soil and can probably be regarded as characteristic members of the soil microflora. Their frequent occurrence on dung is simply a reflection of the fact that this material provides an exceptionallj^ favorable substrate for fructification. No reallj' satisfactory method of estimating their numbers is available; Singh (loc. cit.) reported from 2,000 to 76,400 myxobacters per gram of soil, but this is undoubtedly an underestimate. Two aquatic fruiting myxobacters are known: one is parasitic on an aquatic alga (Geitler, Arch. f. Protistenk., 50, 1924, 67), and the other is an important pathogen of fresh-water fishes (Ordal and Rucker, Proc. Soc. Exper. Biol. Med., 56, 1944, 15). Of the non-fruiting myxobacters, some are common in soil and others are marine forms. Nutrition and cultivation. The ability to decompose complex polysaccharides such as cellulose, agar and chitin is characteristic of many species in the family Cytophagaceae and the genus Sporocytophaga. A few species of fruiting myxobacters belonging to the genera Sorangium, Polyangium and Angiococcus are known to be cellulose-decomposers. For all these forms, the methods of isolation and cultivation are accordingly well-defined and rela- tively simple. Most of them can be enriched from natural sources by the use of a simple mineral base supplemented with the appropriate polysaccharide and thereafter maintained in pure culture on .similar media (KrzemieniewskaandKrzemieniewski, Bull. Int. Acad. Pol. Sci.Lett., Classe Sci. Math. Nat., B, 15, 1937, 11; Stanier, Bact. Rev., 6, 1942, 143; Stanier, Jour. Bact., 53, 1947, 297). A few species appear to have complex growth factor require-
