In the insect Oncopeltus (Hemiptera, Lygaeidae), after blastoderm formation, labeled cells in one segment never give rise to cells in another; clones always respect a sharply defined line, the segmental boundary. Similarly, demarcation lines defining "compartments" have been recently found within the imaginal disks of Drosophila and promise to be of first importance in developmental genetics. In Oncopeltus the segmental border is a straight line in a smiple epithelial monolayer and is marked by a change in pigmentation that is visible in the electron microscope. Reconstructions from serial sections show that there is a change of cell shape at the boundary, but attachment desmosomes, septate junctions, and gap junctions link cells of different segments as well as cells of the same segment. The form of the epithelium at different stages of the molt cycle is described, and the possibility that there may be an abrupt change of cell adhesiveness at the segment boundary is discussed.

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