Fine structural study of interdigital membranes during formation of the digits of the duck foot reveals that the interdigital necrosis is accompanied by a high deposition of collagenous material in the epithelio—mesenchymal interface, rupture of the basal lamina and detachment of ectodermal cells into the amniotic sac. These changes are similar to those observed in the regressing interdigital membrane of the chick although their intensity and temporal extension are less pronounced in the duck. It is suggested that these changes account for the disappearance of the marginal zone of the duck interdigital membranes. The possible causal relationship between the different structural changes are discussed.

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