One of the early ways by which man learns about the sur­ rounding world is by taking to pieces an object which attracts his attention. This method, which is widely used by children, is re­ sorted to once again by the adult who wishes to study natural phe­ nomena. The method of dismantling a complete object into its parts so that it can be studied has found its widest and most suc­ cessful application in biology and, in particular, in the study of living cells. The cytologist studying a cell will usually have to examine its fragments, which may be dead or semiliving. In sec­ tions of killed cells, with the aid of the light or electron micro­ scope he tries to obtain some idea of the structure of the living cell. As he investigates fractions isolated from a tissue homog­ enate, or substances isolated from individual cells, he tries to discover the biochemical functions of the cell organoids and their components. In every case the study of degradation products of the cell (depending on the degree of degradation these may be iso­ lated nuclei, nucleoli, nucleolar ribosomes, ribosomal proteins and RNA, mitochondria, fragments of mitochondrial membranes, and so on) makes its own contribution to knowledge, and the infor­ mation obtained on different objects is mutually complementary. Among all the cell fragments obtaine,d experimentally which can make a great contribution to our knowledge of cell structure, an important place is occupied by contractile cell models.

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I. The Concept of Cell Model. The General Characteristics of Models, Principles of Their Preparation, and Their Sphere of Applicability.- II. Properties of Various Motile Models.- 1. Contraction evoked by ATP.- 2. Lengthening induced by ATP.- 3. Movements composed of a single combination of ATP-induced contraction and relaxation.- 4. Contraction induced by calcium ions and lengthening induced by ATP.- 5. Lengthening induced by calcium ions and inhibited by ATP.- 6. ATP and cytoplasmic streams in plant cells.- 7. Rhythmic movement of flagella and cilia.- III. The Use of Motile Models to Study Problems of a Nonmechanochemical Nature.- IV. Methods of Obtaining Motile Models.- Preliminary remarks.- Actomyosin threads.- Models of the muscle fiber.- Isolated myofibrils.- The muscle fiber without its sarcolemma.- Muscle fiber with disrupted coupling in the sarcotubular system.- Preparation of isolated protofibrils.- Preparation of isolated sarcolemma suspension.- Preparation of relaxing granules from skeletal muscles.- Cell (interphase) models becoming rounded in the presence of ATP.- Anaphase models of fibroblasts.- Models of cleavage in the sea urchin egg.- Telephase models of fibroblasts.- Models of mitochondria.- Models of chloroplasts.- Models of locust and frog spermatozoa.- Models of mammalian spermatozoa.- Models of spermatozoa of certain marine invertebrates.- Models of ciliated epithelium.- Models of the Vorticella stalk.- Model cilia from infusorians.- Models of flagella of unicellular algae.- Models of flagella from protozoans.- Models of amebas.- Protoplasmic streaming models in the marine alga Acetabularia calyculus.- Models of the myxomycete Plasmodium.- The use of models for practical work with students.- ATP and muscle contraction.- ATP and ciliary movement.-Notes Added in Proof.- Surveys.- Experimental papers.