Fig. 333. Head of embryo Skate, 11⁄3 in. long. (From Parker.)
Tr. trabecula; Pl.Pt. pterygo-quadrate bar; Mn. mandibular bar; M.Pt. metapterygoid cartilage; H.M. hyomandibular; Hy. remainder of hyoid arch; Br. 1. first branchial arch; Sp. mandibulohyoid cleft or spiracle; Pn. pineal gland; Au. auditory vesicle; C. 1, C. 2, and C. 3. vesicles of the brain.
The development of the hyoid and mandibular arches in the Skate is characterised by a few important features ([fig. 333]). The anterior element of the hyoid arch, which forms the hyomandibular (H.M), becomes entirely separate from the posterior part of the arch, and only serves to support the jaws. The posterior part of the arch (Hy) carries on the respiratory functions of the hyoid, and is closely connected with the first branchial arch. The upper or metapterygoid element of the mandibular arch (M.Pt) has a considerable development, and, becoming separated from the remainder of the arch, forms a mass of cartilage with one or two branchial rays, in the front wall of the spiracle, and constitutes a section of the mandibular arch still retaining traces of its primitive function in supporting the wall of a branchial pouch.
Although the development of other Elasmobranch types is not known, it is necessary to call attention to the mode of support of the mandibular arch in certain forms, notably Notidanus, Hexanchus and Cestracion, where the pterygo-quadrate region of the mandibular arch is directly articulated to the cranium between the optic and trigeminal foramina. In the two former genera the metapterygoid region of the arch is moreover continuous with the pterygo-quadrate, and articulates with the postorbital process of the auditory region of the skull. In spite of these attachments the mandibular arch continues to be partially supported by the hyomandibular. The skulls in which the mandibular arch has this double form of support have been called by Huxley amphistylic.
Fig. 334. Cranial skeleton of a Salmon fry, second week after hatching; membrane bones, eyeballs, and nasal sacs removed. (From Parker.)
T.Cr. tegmen cranii; S.Or. supraorbital band; Fo. superior fontanelle; Au. auditory capsule; Pa.ch. parachordal cartilage; Ch. notochord; Tr. trabecula; above the trabecula, the interorbital septum is seen, passing into the cranial wall above and reaching the supraorbital band; H. optic foramen; V. trigeminal foramen; l1, l2. labial cartilages; Pl.Pt. palatopterygoid bar; M. Pt. metapterygoid tract; Qu. quadrate region; Mck. Meckelian cartilage; H.M. hyomandibular cartilage; Sy. symplectic tract; I.Hy. interhyal; C.Hy. ceratohyal; H.Hy. hypohyal; G.Hy. glossohyal; Br.1. first branchial arch.
Considering the in many respects primitive characters of the forms with amphistylic skulls it seems not improbable that they preserve the original mode of support of the mandibular arch; from which differentiations in two directions have taken place, viz. differentiations in the direction of a complete support of the mandibular arch by the hyoid, which is characteristic of most Elasmobranchii and, as will be shewn below, of Ganoidei and Teleostei; and differentiations towards a direct articulation or attachment of the mandibular arch to the cranium, without the intervention of the hyoid. The latter mode of attachment is called by Huxley autostylic. It is found in Holocephala, Dipnoi, Amphibia and the Amniota.
Teleostei. In addition to that of Elasmobranchii, the skull of the Salmon is the only hyostylic skull in which, by the admirable investigation of Parker (No. [451]), the ontogeny of the hyoid and mandibular bars has been satisfactorily worked out. Apart from the presence of a series of membrane bones, the development of these bars agrees on the whole with the types already described.