SEQUENTIAL INDUCTION OF THE PRESUMPTIVE EPIDERMIS OF THE RANA PIPIENS GASTRULA

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Abstract
A sequence of inductions of different cell types has been obtained with a single inductor applied to the presumptive epidermis in various concentrations and for varying lengths of time. A neural inductor, such as magnesium sulfate or calcium chloride, will induce a part of the sequence such that a sub-minimal exposure to lithium chloride then is able to induce pigment cells. After lithium chloride is used to induce pigment cells, a strong neural inductor such as sodium bicarbonate, magnesium sulfate or calcium chloride applied immediately after lithium treatment may reverse the induction and nerve cell differentiation will result. The sequence of inductions is a necessary one, as evidenced by the fact that at the peak of competence for pigment cell induction, lithium chloride first induces nerve cells and then pigment cells. The relations of the concept of sequential induction to control of gene action and the organizer phenomenon are discussed.

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