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Re: vertebrate numbers of digits

>Jeffrey martz wrote:

>It is a puzzle to me why tetrapod lineages have often reduced their numbers of 
>digits, but very rarely increased them.  (Icthyosaurs are the only group I
>of.)  Polydactyly (additional fingers) occurs in humans and other tetrapods
>often results in few if any ill effects.  So it is surprising that this 
>evolutionary avenue has so rarely been explored.

I've looked for my old embryology text without success.  However, if
remember correctly, a partial answer lies in the way that limbs and digits
develop.  They begin with a relatively undifferentiated bud of rapidly
dividing mesenchymal tissue.  The bud differentiates and gets its
orientation from biochemical gradients surrounding the bud.  At the time I
studied this stuff (no, I won't say how long ago), the nature of those
gradients was in hot dispute, but their existence was undoubted.
Introduction of additional digit buds at the terminus of the developing limb
would be likely to interfere significantly in the fairly sensitive chemical
geometry required for digit formation. By contrast, elimination of a digit
bud would simplify the developmental mechanics.  

(I'm sure I had that book somewhere.  Perhaps the papyrus has crumbled into
dust ...)

  --Toby White