[Divided nevus--an embryological experiment of nature].

A "divided naevus" (or "kissing naevus") is defined as a congenital melanocytic naevus that occurs on adjacent parts of the upper and lower eyelid and may give the appearance of a single lesion when the eye is closed. We report on four cases of this rare naevus. Two of these were examined histologically and showed the features of compound naevi. The unique occurrence in nature of the divided naevus allows conclusions on the site and time of origin not only of the divided naevus, but possibly also of other congenital melanocytic naevi. The divided naevus must originate at the latest during the period of lid fusion, between the 9th and 20th week of gestation. The fact that the fusion involves only epithelial tissue suggests an epithelial origin of congenital melanocytic naevi, which may later develop secondarily into compound or intradermal naevi by way of "dropping-off".