[Limbs of tetrapods--an actual problem of evolution]

The tetrapod limb is among the most intensively studied developmental systems in tetrapods. Earlier models based on a symmetrical determination of the early limb bud were recently questioned by new observations documenting a predominance of the postaxial axis during limb development. These findings are at variance with observations on limb reduction in a number of fossil reptile clades. Recent investigations corroborate the hypothesis of a decoupling of the patterns and sequence of chondrification and ossification in limb development. Ontogenetic repatterning may establish new topological patterns of connectivity among constituent elements in a biological structure as cartilage is replaced by bone. To view chondrification and subsequent ossification of cartilaginous precursors as decoupled processes bears directly on conjectures of homology of limb elements, and also requires the distinction of two separate mechanisms of limb reduction: truncation of morphogenesis and failure of ossification.