Removal of the block to self-fertilization in the Ascidian Ciona

Abstract

The eggs of Ciona intestinalis found on the Atlantic and Pacific coast of this country cannot be fertilized, as a rule, by sperm of the same individual. A similar phenomenon has long been known in flowering plants. In the latter case it has been shown that the block is due to factors in the pistil that retard the growth of the pollen tube; or else failure to self-fertilize is due to the absence of factors that cause an acceleration of growth in cross-fertilization. In Ciona the nature of the block was unknown despite many attempts to determine it. During the past summer at Woods Hole I have succeeded in bringing about self-fertilization in Ciona by the simple process of freeing the egg from its membranes. The eggs were taken from the oviduct, put into sea water in a flat bottomed watch glass, and by means of two very fine needles the membrane around the egg was torn open. Sometimes the entire egg squeezed out of the slit, sometimes only a part of it pushed out. In either case the extruded protoplasm rounded up into a sphere.