A large dorsal process of the primitive pterygo-quadrate now forms a large metapterygoid tract (M.Pt); while the whole arch becomes firmly bound to the hyomandibular (H.M).
In the later stages the parts formed in cartilage become ossified ([fig. 335]). The palatine is first ossified, the pterygoid region of the pterygo-quadrate is next ossified as a dorsal mesopterygoid (m.pg) and a ventral pterygoid proper (pg). The quadrate region, articulating with the Meckelian cartilage, becomes ossified as a distinct quadrate (qu), while the dorsal region becomes also ossified as a metapterygoid (mt.pg).
In the Meckelian cartilage a superficial ossification of the ventral edge and inner surface forms an articulare (ar); but the greater part of the cartilage persists through life.
Some of the above ossifications, at any rate those of the palatine and pterygoid, seem to be started by dental osseous plates adjoining the cartilage. They will be spoken of further in the section dealing with the membrane bones.
Amphibia. The development of the autostylic piscine skulls has unfortunately not yet been studied; and the most primitive autostylic types whose development we are acquainted with are those of the Amphibia; on which a large amount of light has been shed by the researches of Huxley and Parker.
The modifications of the hyoid arch are comparatively simple and uniform. It forms a rod of cartilage, which soon articulates in front with the quadrate element of the mandibular arch, and is subsequently attached by ligaments both to the quadrate and to the cranium. In those Amphibia in which external gills and gill clefts are lost, it fuses with the basal element of the hyoid ([fig. 330]), which, together with the basal portions of the following arches, forms a continuous cartilaginous plate. On the completion of these changes the paired parts of the hyoid arch have the form of two elongated rods, known as the anterior cornua of the hyoid, which attach the basihyal plate to the cranium behind the auditory capsule.
It is still uncertain whether there is any distinct element corresponding to the hyomandibular of fishes.
Parker holds that the columella auris of the Anura is the homologue of the hyomandibular. The columella develops comparatively late and independently of the remainder of the hyoid arch, but the similarity between its relations to the nerves and those of the hyomandibular is put forward by Parker as an argument in favour of his view. The early ligamentous connection between the quadrate and the upper end of the primitive hyoid is however an argument in favour of regarding the upper end of the primitive hyoid as the hyomandibular element, not separated from the remainder of the arch.
The history of the mandibular arch is more complicated than that of the hyoid. The part of it which corresponds with the upper jaw of Elasmobranchii exhibits most striking variations in development; so striking indeed as to suggest that the secondary modifications it has undergone are sufficiently considerable to render great caution necessary in drawing morphological conclusions from the processes which are in some instances observable. A more satisfactory judgment on this point will be possible after the publication of a memoir with which Parker is now engaged on the skulls of the different Anura.
The membrane bones applying themselves to the sides of the mandibular arch are relatively far more important than in the lower types. This is especially the case with the upper jaw where the maxillary and premaxillary bones functionally replace the primitive cartilaginous jaw; while membranous pterygoids and palatines apply themselves to, and largely take the place of, the cartilaginous palatine and pterygoid bars.