A NEW THEORY OF LIMB INDUCTION

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Abstract
Tracer studies on cells of Amblystoma punctatum embryos undergoing limb induction. Limb development can proceed only when there is a harmonious interaction between the mesoderm and epithelium of a limb rudiment. In this interaction the mesoderm not only causes the epithelium to react but is itself controlled by an influence exerted by the epidermis. Stimuli of an inductive nature thus have to pass both ways between the mesoderm and the epithelium. Transmission of an inducing stimulus is in some cases at least (neural-plate induction, lens induction) dependent on a direct contact between the acting and the reacting components. If the same were true in the case of a limb rudiment, the intervention of a well-developed fibrous basement membrane would effectively preclude the possibility of limb development even if the components (mesenchyme and epidermis) were capable of participating in such a process. Induction of a heterotopic limb may then well be a direct result of the absence of the epidermal basement membrane at a particular spot, correctly timed, so that the competent mesodermal cells are able to establish direct contact with the epidermis.

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