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Abstract:
Earlier proposed models for primary pattern formation, for gene activation and for segmentation are summarized and compared with recent molecular-genetic observations. A model for head, foot, tentacle and bud formation in Hydra illustrates that complex patterns can be reliably generated. Stable cell determination requires autocatalytic (autoregulatory) genes. Segmentation in insects has been proposed to result from a reiteration of (at least three) cell states. Their patterning is achieved by a mutual activation of cell states that locally exclude each other. A model for accretion of new segments by proliferation at the posterior pole is proposed that accounts for the generation of a periodic and a sequential pattern in register with each other. The assumption of a process analogous to segmentation in plants can account for the initiation of leaves with an intrinsic polarity that eventually leads to the upper and lower leaf surfaces. The model accounts also for the formation of axillary buds in correct relation to a leaf and for the much smaller spacing of leaves within a whorl when compared with the spacing between two successive whorls along the shoot. It is concluded that the generation of complex structures in distantly related organisms may be based on similar mechanisms.
