Branch initiation by global control
In the mechanisms discussed so far, only information exchange between neighbouring cells has been considered. However, several patterns indicate that particular events occur simultaneously at very distant positions. For instance, the shell of Oliva porphyria (Figure 6.1) shows oblique lines with branching. A branch along an oblique line indicates the sudden formation of a backward wave. Many branches are initiated at the same time at distant locations. The dashed line in Figure 6.2b indicates such a moment. There would not be enough time for a signal to travel over such a long distance by diffusion. On the other hand, not all lines branch at this critical moment. This indicates that from time to time a signal is generated everywhere in the mantle gland that greatly enhances the probability of wave splitting. Such a signal must be the result of a global control.
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