Abstract
Ommatidia appear to use the morphogenetic furrow for anterior- posterior information (the furrow is assumed to be initiated in the pos- terior of the disc) and for dorsal- ventral information (the furrow is assumed to be initiated in the dorsal- ventral midline). Both are potentially .sensed through short-range cell inter- actions 9.ILI2, providing an excellent example of how local interactions can create a long-range pattern. The important thing seems to be that a firing centre provides an initial asymmetry, from which a pattern would then propagate outwards, by morphogenetic furrow movement (anterior-posterior) and by short- range signalling between ommatidia (dorsal-ventral). It makes sense to COMMENT use a feature whose polariw is already defined, but the question now be- comes: what decides that the mor- phogenetic furrow is initiated at the posterior midline? It could be that the initiation mechanism (which is largely unknown) responds to a global co- ordinate system in the disc. Alter- natively, it could be induced by yet another prior feature in the disc, such as the nearby optic stalk (Fig. lc).
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