By studying the pattern of movement of spindle cells in the bone marrow and peripheral blood of chicken by tissue culture, the author obtained the following results. 1) First, the average size of spindle cells is about 8.3×4.2 μ and that of the nuclei about 4.4 μ×3.4 μ. Under the phase-contrast microscope cytoplasm is of thin and irregular composition and the cytoplasmic membrane is indistinct. Nuclei are dense and filled with minute chromatin network and the nuclear membrane is not clear-cut. 2) The author divided the patterns of spindle cell movement into four types; namely, deformation of cell (type A), formation of membraneous projections (type B), trembling movement like Brown's movement (type C), and rotating movement (type D), and 7 subtypes; but all these patterns of movement resemble quite closely to those of normal platelets. 3) Spindle cells with type A movement are most numerous both in bone marrow and peripheral blood; and the maximum number of them appears in the early stage but it decreases along with the lapse of time. Because B and C types of movement appear more frequently in the later stages, A type seems to be the type of movement taken by spindle cells when their function is more active than the time when they take B and C types of movement. 4) Spindle cells of type D movement possessing cell body and nuclei both oval in shape with chromatin scattered around the nuclear membrane can be recognized only in bone-marrow culture but not in peripheral blood culture. Consequently this type is assumed to be an immature type. Moreover, no megakaryocytes can be detected in the bone marrow of chicken so that the author has interpreted those spindle cells belong to the independent cell system which mature from the immature type.