demonstrated clearly the protective value of numbers of animals as compared with single individuals when exposed to equal volumes of colloidal silver suspension of the same concentration. Allee and Schuett ('27) confirmed and extended these experiments. Mass protection from the toxic effects of colloidal silver has been demonstrated by these workers for a wide range of animals, including, among others, various Protozoa, planarians, leeches, ophuroid starfishes, crustaceans, and frog tadpoles. Drzewina and Bohn originally interpreted their results as being due primarily to the more rapid production of some autoprotective substance by the group than by the isolated individuals, which acted by some other means than the fixation of the toxic materials. The experiments of Allee and Schuett indicated that, under the conditions of their experiments, a major part of the protection furnished by the mass was due to the more rapid adsorption of the toxic colloidal silver either directly on the surface of the animals or upon the slime which the animals had shed into the surrounding medium, but their experiments did not prove conclusively that the observed protection was due entirely to such adsorption.

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Heterotypic conditioning of water has long been known to favor the growth and well being of aquatic animals, particularly the type of conditioning which yields balanced aquaria. The addition of other animals is standard practice and is illustrated by the placing of snails in fish culture tanks. Recently, Shaw ('32) has demonstrated that when half the water in experimental and control aquaria is changed daily, filtered mussel-conditioned water favors fish growth more than similarly treated but unfiltered and unconditioned water. Homotypic conditioning, on the other hand, has usually been reported to yield harmful results only.Recent studies, which are summarized elsewhere (Allee, '31, '34) have emphasized the fact that undercrowding is as real a menace as the better known and more easily demonstrated danger of overcrowding. I n this connection, work upon the effect of biologically conditioned water on the rate of growth in fishes was begun in this laboratory in 1926, has continued without interruption to date, and is still in progress. I n the early work (Church, '27; Shaw, '29) the young of Platypoecilus maculatus were placed in 8 liters of Whitman Laboratory well water in assembled glass aquaria in lots of We are indebted t o Dr. Gretchen Shaw for permission to publish the results of experiments I X t o XI, and to Miss Gertrude Evans f o r similar permission regarding experiments XX to XXII; t o Messrs. S. B. Haenisch and E. J.Rosenbaum for chemical aid i n the early stages of the work.

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