Zoo-logioal I a b o r a t o r y , University of Pennsylvania, PhiladelphiaThere are 2 ways to study the colloidal behavior of muscle protoplasm. One way is to isolate individual proteins of the muscle. The colloidal behavior of such proteins can then be followed exactly in test tubes or other suitable containers. In recent years, biochemists have learned to make relatively pure preparations of muscle proteins and the physical or colloidal properties of these proteins have been carefully noted by many investigators (see f o r example Dainty, Kleinzeller, Lawrence, Miall, Needham, Needham and Shen, '44 ; Szent-Gyorgyi, '45, '46). There is also a second way in which the colloidal behavior of muscle may be studied. This consists of a direct investigation of the muscle protoplasm itself.Both types of approach have disadvantages. The exact results obtainable with the purified proteins are satisfying as far as they go, but there is no certainty that the colloidal behavior of protoplasm can be duplicated by the behavior of 1 or 2 of the proteins which can be extracted from it. Indeed all the available evidence indicates that the colloidal behavior of protoplasm is in marked contrast to that of any known protein in the pure state. This is the central thesis of a monograph published by the senior author nearly 20 years ago (Heilbrunn, '28), The primary disadvantage of studying the colloidal behavior of muscle protoplasm directly is that exact methods are difficult. It is not possible to make correct determinations

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