Limb Development in the Human Embryo
Similarities in the choreography of the development lead to an overall similarity in the anatomical structures of tetrapod limbs; they are all composed of the similar three sections: stylopod, zeugopod and autopod. These homologies should not surprise those who follow developmental biology, as many genes that initiate regulatory processes are highly conserved amongst all animals, and anatomical structures are often conserved across phylum and class.
What is needed is an account of biological change that helps to understand the flexibilities inherent in morpho-genic fields as potentials for expression induced by the heterogeneous spaces and times important in the development of bodies. This would also be an account that would pay attention to the different types of chemical competencies expressed by molecules at different types of moments. Not all moments are the same in genetic development, some moments carry greater impact by directing specific types of interactions. For those of us interested in contemplating the importance of the diversity of living things, these moments matter.