ABSTRACT
Electron microscopical study of the formation of the hind limb of the toad Xenopus laevis between stages 49 and 53 of larval development has been performed with special reference to the relationships between the apical ectodermal ridge and the underlying mesenchyme. It has been found that the intervening collagenous layer is thinner in the vicinity of the ridge than elsewhere and that the subjacent mesenchymal cells are more loosely arranged. There are also fewer mesenchymal filopodia crossing the collagenous layer than proximally although the distribution of delicate vertically disposed threads, also observed at the ecto-mesenchymal junction, is uniform over the entire bud. Overall, it is concluded that the fine structural features of Xenopus limb development, during these stages, do not give clear insight into the mechanisms by which the apical region exerts its morphogenetic effects.