Morphogenesis of the carotid labyrinth in the bullfrog, Rana catesbeiana, during larval development and metamorphosis.

Morphogenesis of the carotid labyrinth in Rana catesbeiana during larval development and metamorphosis was studied using vascular corrosion casting and scanning electron microscopy. The carotid labyrinth was formed at the point where the carotid arch descends to the internal gills. The transformation of the appearance of the carotid labyrinth can be summarized in six phases. 1. Through the early stages of larval development (stages I-V), the slightly expanded region of the external carotid artery becomes closely connected with the carotid arch, and these two arteries are joined by small channels on their opposing faces. 2. By the last stage of foot stages (stage XVII), the expanded region becomes globular. 3. At the middle of the metamorphic stages (stage XXII), many protuberances appear on the surface of the globular expansion. 4. At stage XXIII, these form a rudimentary vascular maze. 5. At stage XXIV, this globular expansion is completely surrounded by a simple maze-like structure that resembles the carotid labyrinth in adults. The origin of the internal carotid artery is located within the vascular maze. 6. At the final stage of metamorphosis (stage XXV) the carotid labyrinth is nearly completed. The vascular ring and vascular routes found in the adult can be recognized at the proximal and distal end of the labyrinth, respectively. These findings suggest that the premetamorphic carotid labyrinth has a vascular regulatory function.