An Experimental Analysis of the Role of Cytoplasmic Fountain Streaming in Furrow Establishment

During furrow establishment the equatorial cell surface is altered so that it becomes capable of exerting the physical force required for cytokinesis. It has been suggested that the alteration results from deposition of a furrow-organizing substance at the equator by convergence of “fountain streaming” patterns of cytoplasmic flow. The purpose of this investigation was to determine whether a fountain streaming pattern is essential for furrow establishment. The shape of sand dollar eggs was altered before the time of furrow establishment in ways that would greatly disrupt fountain streaming. Shapes that should have abolished regions of convergence and shapes that should have created several such regions were created by notching and perforating cells. Furrow establishment took place under circumstances that would be highly unlikely if a fountain streaming pattern of cytoplasmic currents were necessary.