X-irradiation of eggs and larvae of Oncopeltus fasciatus results in the development of clonal patches of epidermal cells of unusual pigmentation. The frequency, size and distribution of these patches is dependent on the dose and timing of irradiation. Analysis of these clones in the abdomen has shown that the presumptive epidermis becomes effectively segmented during blastoderm formation and thereafter the clones are restricted to within a segment quadrant (dorsal or ventral, left or right). There are approximately 10 presumptive epidermal cells per segment quadrant. The shape of the clones and the orientation of mitoses in larvae suggest that both early and late cells of the anterior margin of the segments divide with a preferred orientation (cleavage plane parallel to the antero-posterior axis). Elsewhere in the larval segment the mitoses are randomly oriented, and the segment grows evenly all over. The number of mitoses/cell/moult cycle is not precisely determined, but the amount of cell division is perhaps under a general probabilistic control. It is suggested that the segmental gradient may be involved in this control.

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