SummaryThe present review of the literature of mass physiology is limited to the years just previous to 1933, and for approximate completeness must be read in connection with an earlier and more extensive survey (Allee, 1931). Analysis of the reactions leading to the formation of aggregations in nature or in the laboratory has scarcely proceeded beyond the recognition hat much of such behaviour is innate. There is, however, recent evidence that a part of the schooling behaviour of the fishAmeiurusis acquired rather than inherited.Once formed, aggregations of aquatic organisms condition the medium surrounding them by the addition of secretions and excretions, the nature and the biological effects of which form one of the important problems of mass physiology.It is easy to demonstrate that overcrowding lessens the rate of growth of organisms. More recently evidence has been accumulating that undercrowding frequently has the same effect. Evidence is presented on this point in such widely different animals as mealworms, fishes and mice. Similarly, with population growth the harmful effects of undercrowding have recently been found for protozoans, crustaceans and beetles, as well as the ill effects of overcrowding.The results from aggregation upon rate of oxygen consumption varies with different animals, and even with the same animals at different times of the year. With goldfishes, those in small groups use less oxygen per individual if grouped than when isolated; with the more closely schoolingAmeiurusopposite results are reported. Outside the breeding season, the brittle starfishOphiodermaconsumes less oxygen per individual if grouped; this relationship does not necessarily hold during the breeding season.Groups of animals are able to afford protection to their members if exposed to toxic conditions produced either by the absence of accustomed salts, as the marine flatwormProcerodesdoes when placed in fresh water, or by the presence of toxic substances such as colloidal silver. The amount of protection furnished has been measured for certain cases, and the protective mechanisms are discussed.The polarity of the seaweedFucuscan be determined by the position of a given egg with reference to a group of other eggs of the same or of a different species. Such plasticity might be expected from plants more readily than from animals. With animals, Uvarov's phase theory of locusts has been experimentally demonstrated to hold for the South AfricanLocustana pardalina. The transition from parthenogenetic to sexual reproduction in certain cladocerans has again been demonstrated to result from crowding, and considerable progress has been made in the analysis of the relative importance of the presence of metabolic products and of nutrition in the control of sex in these animals.The effects of numbers present upon the rate of learning differs with different animals and even in the same animals with different problems. Thus fishes learn to run a simple maze more rapidly if in groups than if isolated, but they learn less readily to jump for a bit of worm held just above the water level. Cockroaches also learn to run a simple maze more slowly if more than one is present in the maze at the same time.Groups of birds show a fairly definite flock organisation, which may or may not be related to active leadership of the flock.The whole range of mass physiology has been presented with the thought that it forms a large part of the background for social life.