Cytokinesis: Effects of blocks between the mitotic apparatus and the surface on furrow establishment in flattened echinoderm eggs

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Abstract
The physical mechanism which accomplishes cytokinesis in animal cells develops as a consequence of interaction between the mitotic apparatus and the surface. The purpose of this investigation was to determine whether successful furrow establishment depends upon normal geometrical relations between the interactants in some areas but not in others. Blocks were positioned between different regions of the mitotic apparatus and the surface in flattened sea urchin (Tripneustes gratilla) and sand dollar (Echinarachnius parma) eggs. The blocking agents used were slit‐shaped perforations, needles, and oil drops. Results were consistent regardless of which block was used. Blocks located outside the equatorial plane did not affect furrowing. Perforations located in the equatorial plane resulted in furrow formation on the proximal surface of the perforation; no furrows formed distal to the perforation, either in the cell margin or the distal surface of the puncture. Oil drops and glass needles positioned in the equatorial plane affected furrowing. The magnitude of the effect was directly related to the size of the block. The results demonstrate that furrow establishment is not affected by drastic modifications of the geometrical relationship between the mitotic apparatus and the poles, but it is very susceptible to interference in the region between the mitotic apparatus and the equator.