Fig. 1. The findings by D’arcy Thompson and their implications.

A D’arcy Thompson demonstrated that the shapes or spatial arrangement of anatomical landmarks of related animals can be mapped onto each other by relatively simple geometrical transformations (left panels). His study suggested the existence of a standard shape $\tilde{\mathrm{S}}$ across a group of species from which the shape of each species $S_i$ is derived. Denoting it as $S_i={\sigma }_i\left(\tilde{S}\right)$, Thompson’s work states that the interspecies transformation ${\mathbf{A}}_{ij}={\sigma }_j\circ {\sigma }_i^{-1}$ could be relatively simple. The fish silhouettes were traced from diagrams in Thompson’s work¹. B Hindlimb morphogenesis in chick and Xenopus laevis. The Xenopus limb bud is about half the size of that in chick for each axial direction (note that the scale bar is common for both species). Also, the anatomical regions formed from the initial limb buds differ between the two species. In Xenopus, all limb structures, i.e., the autopod (A), zeugopod (Z), and stylopod (S), are formed from the limb buds (the dotted region in a limb bud from St. 50.5 shows the prospective autopod and zeugopod regions). In chick, only the former two are formed, and the stylopod is embedded in the trunk. Skeletal patterning with cartilage differentiation becomes obvious around St. 54 for Xenopus and St. 30 for chick. The proportion of each skeletal segment (in particular, the tibia/fibula and tarsal) and the number of digits differ between these species. White arrow heads: hindlimb bud.