The palatopterygoid soon becomes segmented into a transversely placed palatine, and a longitudinally placed pterygoid ([fig. 340]). With the exception of a few ossifications, which present no features of special interest, the parts of the mandibular arch have now reached their final condition, which is not very different from that in the Axolotl.
Sauropsida. In the Sauropsida the modifications of the hyoid and mandibular arches are fairly uniform.
The lower part of the hyoid arch, including the basihyoid, unites with the remnants of the arches behind to form the hyoid bone, to which it contributes the anterior cornu and anterior part of the body.
The columella is believed by Huxley and Parker to represent, as in the Anura, the independently developed dorsal (hyomandibular) element of the hyoid, together with the stapes with which it has become united[205].
The membranous mandibular arch gives off in the embryos of all the Sauropsida an obvious bud to form the superior maxillary process, and the formation of this bud appears to represent the growth forwards of the pterygoid process in Elasmobranchii, which is indeed accompanied by the formation of a similar bud; but the skeletal rod, which appears in the axis of this bud, is as a rule independent of that in the true arch ([fig. 331], pa, pg). The former is the pterygo-palatine bar; the latter the Meckelian and quadrate cartilages.
The pterygo-palatine bar is usually if not always ossified directly, without the intervention of cartilage.
Born has recently shewn that Parker was mistaken in supposing that the palatopterygoid bone is cartilaginous in Birds. In the Turtle a short cartilaginous pterygoid process of the quadrate would seem to be present (Parker, No. [458]).
The quadrate and Meckelian cartilages are either from the first separate, or very early become so.
The quadrate cartilage ossifies as the quadrate bone, and supplies the permanent articulation for the lower jaw. Its upper end exhibits a tendency to divide into two processes, corresponding with the pedicle and otic processes of the Amphibia. The Meckelian cartilage becomes soon covered by investing bones, and its proximal end ossifies as the articulare. The remainder of the cartilage usually disappears.
Mammalia. The most extraordinary metamorphosis of the hyoid and mandibular arches occurs in the Mammalia, and has been in part known since the publication of the memoir of Reichert (No. [461]).