Fig. 4

SEM of Chamaeleo calyptratus hand and foot morphogenesis. Early stages of morphogenesis show that the chameleon hand develops as a round digital plate (a) which subsequently develops a distal flattening (d, g). In dorsal view, these stages present a very robust AER, relatively larger than that present in A. uniparens (Fig. 3) while in distal view the AER is seen as having a greater thickness along the dorsoventral midline (b, e, h). Significantly, during the stage at which the distal autopodium begins to flatten (g), the distal AER is no longer the stereotypical A-P flattened ectodermal thickening but is instead arched ventrally (h, i). Surprisingly, despite having a robust distal AER, proximal mesenchymal cleft formation has already begun (j, k). At later stages (l-o, p-s), both the forelimb and hindlimb expand the cleft while maintaining the AER quite robust until the thickness tapers and narrows at later stages (while the AER also returns to its expected conformation of a straight distal ridge)