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Herbst (1892) made the remarkable discovery that the differentiation of the sea-urchin egg was shifted in a vegetal direction when lithium ions were present in the sea-water. The vegetalization implies a failure of the apical tuft to appear, a displacement of the skeleton-forming cells in the animal direction, a reduction of the ectodermal region, and a corresponding enlargement of the endoderm, which often leads to exogastrulation. Strong lithium action may lead to complete endodermization of the egg. Many other substances have later been found to cause a change of differentiation, either in a vegetal or animal direction, e.g. a partial or complete animalization by treatment of unfertilized eggs with SCN- or 1-ions (also SO4, Br, and tartrate, Lindahl, 1936). The animal and vegetal principles are considered as two opposite, antagonistic gradients (Runnström, 1928 a, b) representing different types of metabolism (Lindahl, 1936).

Vegetalization of the Sea-Urchin Egg by Dinitrophenol and Animalization by Trypsin and Ficin