Size dependence during the development of the amphibian foot. Colchicine-induced digital loss and reduction.

Localized treatment of the limb buds of the frog, Xenopus laevis, and the salamander, Ambystoma mexicanum, with the mitotic inhibitor colchicine results in limbs that, when compared with the contralateral control, are smaller in size and have lost skeletal elements. There is a very well defined pattern in terms of what elements are most likely to be lost. For example, frogs that have lost a toe always lose the first toe, while salamanders always lose the fifth. These differences correspond to qualitative differences in developmental sequence of digital differentiation in anurans as compared to urodeles. We propose a hypothesis in which the digital pattern is indirectly affected by reduction in the number of mesenchymal cells in the embryonic field.