ABSTRACT
In the legless lizard, Ophisaurus apodus, the hind-limb primordium appears on the caudal extremity of the Wolffian ridge at an early stage in the development of the embryo (4·2 mm long). Three somites each send an extension into this young bud and distribute cells in its mesenchymal mass of cells. An apical epiblastic ridge, appearing as a fold from the exterior, forms on the limb-bud, which at this time is flattened at the distal end. A large vessel and nerves penetrate into the bud which appears normal at this stage. Necrosis then begins, first in the ridge, then in the cells of somitic derivation and finally, when the ridge has almost disappeared, in the mesenchymal cells. The bud which has been growing up till now, starts to regress at this point. It does not, however, disappear completely as it does in the forelimb, but remains as a rod-like appendage on each side of the cloaca. Inside this structure a rudi-mentary skeleton takes shape. The phallic primordium which appears on the ventro-caudal aspect of the hind-limb bud is not involved in this degenerative phenomenon and continues to develop.
The arrest of development may be due to the degeneration of the apical ridge; possibly resulting from a failure of induction by the somites or to an insufficient somitic contribution to the hind-limb bud.
