THE DEVELOPMENT OF THE SKULL OF GAMBUSIA-AFFINIS-AFFINIS (BAIRD AND GIRARD) .2. THE DEVELOPMENT OF THE MANDIBULAR ARCH

The ontogeny of the mandibular arch of the teleost fish, Gambusia affinis affinis is studied in six developmental stages and compared with that of other bony fishes. The mandibular arch is composed of the palatoquadrate and Meckel's cartilage; the two elements appear earlier during ontogeny as independent structures in a procartilaginous state. The pterygoid part of the palatoquadrate develops as an anterior outgrowth from the quadrate part in the 4 mm larval stage. The palatoquadrate articulates with the ethmoid plate of the chondral neurocranium by an ethmopalatine articulation in the 6 mm stage. Meckel's cartilage first appears at the 3-5 mm stage anteriorly, separate from its fellow of the opposite side. At the 5 mm larval stage the two Meckel's cartilages connect with each other. At the 6 mm larval stage the posterior end of each cartilage is differentiated into two processes; the coronoid and retro-articular processes. The palatoquadrate articulates with Meckel's cartilage between these two processes.