Patterns of embryonic development

The embryology of reptiles is an important component of two emerging research areas. One area is the result of the marriage of molecular and developmental biology. Molecular biology has provided the tools to demonstrate the genetic commonality of developmental patterns, and thus provides novel insights into the origin and maintenance of structural diversity across the animal kingdom (Raff, 1996). Hox genes are the master regulatory genes whose products are expressed in specific locations on the long axis of embryos and so are the cornerstones of this commonality. Hox genes, and the cascade of other regulatory and structural genes that they affect, specify the axial pattern (trunk regionalisation and limb position) during embryonic development. Mutations to Hox genes, thus, can alter the number and position of structures and even determine whether some particular structure develops at all. For example, studies on limb development have demonstrated that modifications to Hox and associated regulatory genes provide a mechanistic explanation for the absence of forelimbs and the extreme reduction in size of the hindlimbs of pythons, and squamates with other reduced limbs (Cohn and Tickle, 1999).