In all eukaryotic cells the movements of cytoplasm can be observed at some stage during development. Such movements may be involved either in translocation of cytoplasmic components within the cell, or in the generation of forces resulting in the movement of the cell over the substratum. Recent biochemical studies have demonstrated the existence of at least two classes of cellular structures which are involved in generating movements in a wide variety of cell types1, 2, 3, 4. The first of these systems involves the interaction of microfilaments composed of the proteins actin and myosin, resulting in local contractions5 — a mechanism similar in many essential features to the contraction of vertebrate striated muscle6. A second major kind of motile machinery is based on the interaction of microtubules3,7 either with one another or with microfilaments.