Fig. 4. Loss and overexpression of omb. The diagrams summarise the model. The blue line traces the actual concentration profile of X, with the brown shading indicating the strength of X production. The slope of X concentration is the vector which defines planar polarity at each point. (A) In the wild type the highest concentration of X and the highest rate of production of X coincide in cells at the back of the A compartment. (B) A clone of omb^– cells at the back of the A compartment produces little or no X, and so X spreads into it from the front, forming a reversed gradient and reversed polarity. (C) A clone of omb^– cells positioned more anteriorly than the clone in (B), X is produced both in front and behind the clone, creating two peaks and producing a reversed gradient that begins within the clone and extends anteriorly. (D) A clone over-expressing X will make a local peak of X, causing reversal within the back of the clone and behind it.