The skeleton, or endo-skeleton, assumes a vast importance in Vertebrata. It consists of an axial and an appendicular portion. The former includes the skull and backbone: the latter the skeleton of the paired limbs and the girdles to which they are attached.
The notochord, which forms the axis of the backbone, extends into the skull as far forwards as the pituitary fossa. It undergoes atrophy, as a rule, in the skull, even though it persists, more or less, in the backbone. Exceptions are Cyclostomi and the Sturgeon (Acipenser). The skull itself is derived from the following elements: (1) two parachordal cartilages which lie one on each side of the anterior end of the notochord; (2) two trabeculae cranii underlying the fore-brain, and in shape resembling more or less lyriform rods in contact near their anterior extremity, and diverging posteriorly where they encircle the pituitary fossa and come into contact with the two parachordals 1 with which they are continuous in development in higher types (Bird, Pig), and with which they ultimately unite in all classes. They fuse together anteriorly in the region of the nose except in Eels. They retain in Ophidia the form of two rods in the region between the nose and the parachordals. Except in Urodelous Amphibia and a few other forms, the membranous floor of the pituitary region chondrifies continuously with the trabeculae, and is perforated only by the carotid arteries. Chondrification extends upwards from the parachordals and trabeculae in the layer of indifferent cells investing the brain.
The former constitutes the occipital region. The trabeculae give origin to the basi- and alisphenoid, the paired prae- and orbito-sphenoid; the interorbital septum, when it is present((Teleostei, Lacertilia, Aves); the paired lateral or ecto-ethmoids in front of the orbits; the internasal septum; and sometimes a prae-nasal rostrum, as in many Elasmobranchii, etc. The degree to which the cranial cartilage extends dorsally varies. The part of the chondro-cranium formed as above becomes (1) united with the cartilaginous capsules of the nose, and ear, that of the eye remaining independent, and (2) connected to visceral arches. The capsule of the nose is open externally and coalesces with the ethmoidal region. The capsule of the ear is generally closed, except where the auditory nerve enters it, and when two membranous spots, the fenestrae ovalis and rotunda, are formed on its external face. This capsule makes part of the lateral wall of the chondro-cranium and is wedged in between the occipital and basi-sphenoidal regions. It may even intervene in the floor of the skull between the two.
The visceral arches, which are independent of the chondro-cranium, are paired and post-oral. The first is the mandibular or Meckel's arch, the second the hyoi-dean, the third and succeeding are branchial arches. The hyoidean and branchial arches are united ventrally by basal pieces or copulae, not, however, equal in number, to the arches themselves. Meckel's arch in some Elasmobranchii and in Anura, among Amphibia, bends over the angle of the mouth. The piece bent over lies in front of the mouth, and is cut off, forming the palato-pterygo-quadrate bar, whilst the remaining part of the arch forms the cartilaginous mandible. In other Vertebrata the palato-pterygoid is an independent formation, and Meckel's arch is segmented transversely, forming a small upper segment - the quadrate - or in Teleostei and bony Ganoidei, the quadrato-metapterygoid, and a large lower segment - the cartilaginous mandible. The hyoidean arch also undergoes transformation. Its upper part is cut off as a hyomandibular-symplectic in Pisces, or a stapedial element in Amphibia and higher Vertebrata, while the lower part of the arch forms the hyoid proper.
This hyoid in the limited sense, proceeding from its dorsal to its ventral end, is broken up typically into stylo-, epi-, and cerato-hyal divisions, to which, in some Fish, is often added a hypohyal. A branchial arch is typically divided in the same manner, but the posterior arches in the series are simpler in structure1
1 It is possible that the trabeculae are dissociated portions of the parachordals.
With the addition of a variable number of labial cartilages round the mouth, the above constitutes the cartilaginous skull. When it undergoes a normal ossification the occipital region divides into a basi-, a right and left ex-, and a dorsal supra-, occipital bone. The ear-capsule contains at the most five bones, pro-, sphen-, pter-, ep-, and opisth-, otics, but the second and third of the five are frequently absent. A basi- and two ali-, sphenoids appear in front of the ear-capsule, and in front of them in turn a prae- and two orbito-, sphenoids. The nasal septum ossifies as a mesethmoid and the lateral masses each as an ecto-ethmoid bone. A palatine, pterygoid, and sometimes a meso-pterygoid appear in the palato-pterygoid bar. The quadrate region forms the quadrate bone, and in Teleostei and bony Ganoidei a meta-pterygoid also. The mandible has generally an articular ossification at its proximal end and occasionally an angular. An ossification sometimes found at the distal end of each mandible, and known as Mento-meckelian, is a persistent lower labial cartilage.
The hyomandibular element of the hyoid arch has, in Teleostei and bony Ganoidei, two ossifications - the hyomandibular and the sym-plectic. It forms the simple stapes of some Amphibia and Reptilia and the Mammalia; the complex chain of stapedial elements (medio-, extra-, infra-stapedials) of Anuran Amphibia, the Crocodile, and Aves. The other segments of the hyoid ossify separately, but are sometimes more or less represented by ligament. The segments of the branchial arches may be similarly ossified. In adult Amniota the first only is represented, and is large in some Reptilia and in Aves; the rest abort.
1 Dohrn has recently investigated the embryoes of various Elasmobranchii, and has come to the conclusion that (1) the palato-pterygoid, (2) the mandible, (3) the spiracular cartilage, (4) the hyomandibular, and (5) the hyoid represent as many visceral arches. With the exception of the hyoid, these arches remain simple. The hyoid segments and its dorsal element fuses with the hyomandibular in Selachoidei. The thyroid gland is, according to him, the remnant of the gill clefts between the hyomandibular and hyoid arches. See Mitth. Zool. Stat. Naples, vi. 1885.