To this cartilaginous skull, as above-described, are added a series of bones, developed in the first instance from the skin or mucous membrane of the mouth, but engrafting themselves in higher forms upon the skull or the cranium proper. The principal bones in this series, often spoken of as 'membrane' bones, are paired parietals, frontals, nasals, lacrymals, upon the dorsal aspect of the cranium; a vomer single or double, and except in Mammalia, where it is scarcely identifiable, a parasphenoid, on its oral aspect. To the palato-pterygoid bar are added praemaxillae, maxillae, jugals, and sometimes quadrato-jugals. To the quadrate region belongs the squamosal; to the mandible a dentary, and in Sauropsida a splenial, coronoid, angular, and supra-angular. A tympanic bone underlying the tympanic cavity of the ear appears in Mammalia, and in many Fish the opercular flap of integument covering the branchial cavity contains opercular bones: see p. 93.
The backbone is formed by the notochord and its sheath alone, in Myxinoidei; with the addition of neural arches in Petromyzon, and of haemal arches as well in some Pisces (certain Elasmobranchii, chondrostean Ganoidei). In all other Vertebrata the notochord is constricted by vertebral centra, and except in Pisces, some Amphibia and Reptilia, but slight traces of it persist in the adult in the intervertebral regions. The arches are developed in mesoblast independently of the notochordal sheath in Ichthyopsida, in continuity with it in other Vertebrata. The neural arches originate from a continuous right and left ridge in most instances; they inclose the spinal cord, and in Elasmobranchii form a complete cartilaginous investment, which is segmented into neural arches proper, placed vertebrally, and intercalary pieces placed intervertebrally. The intervertebral regions are occupied by a fibrous membrane in other Vertebrata, and it is only in a few instances that intercalary pieces are rudimentarily present. Haemal arches are well-developed in Pisces, where they originate like the neural arches from a continuous right and left ridge. Haemal intercalary pieces are present in the tail of Elasmobranchii, but not in the dorsal region.
Haemal arches are developed also in the tail of Urodelous Amphibia. How far the transverse processes ( = di- and par-apophyses) of the vertebrae in other Vertebrata can be said to represent them is doubtful. The processes in question are continuous both with the centrum and the neural arch, or with one of the two in the adult. They are said to arise independently and then to fuse with the vertebrae in Urodelous Amphibia; in other cases they appear to be out-growths from them. Vertebral centra are formed by the growth of the cartilaginous or cellular sheath of the notochord, together with additions from the bases of the arches in Elasmobranchii, or from the surrounding mesoblast in Teleostei. The centrum is primitively due to a growth of cartilage in the vertebral region, and so it remains in Pisces and Mammalia, with isolated examples, living or extinct, in other classes. Large intervertebral remnants of notochord persist in Pisces; very slight in Mammalia. Traces of the vertebral growth may be visible in the development of Amphibia and Sauropsida, but they are masked by a great growth of cartilage in the intervertebral region. The vertebrally placed remnants of notochord are then converted into cartilage and very commonly eventually into bone.
The vertebral centrum may be biconcave (amphicoelous), biconvex, concave in front or behind ( = pro- and opistho-coelous), or flat. It may remain independent of the arches or fuse with them. It is always ossified, and ossification may spread from it to the arches; or the latter may ossify separately. The centra alone may be connected together (most Pisces, a few Reptilia) or articulating processes ( = zygapophyses) may be developed from the neural arches both from their anterior and posterior aspects, as in Amphibia and higher Vertebrata. The neural arches are generally provided with a dorsal neural spine (neurapophysis), formed independently or in continuity with them; and the centra may have a ventral outgrowth or spine (=hypapophysis). Each vertebra, consisting of centrum and neural arches, corresponds to a myocomma; the intervertebral region to the centre of a myomere. However in the tail of some Elasmobranchii the centra are twice as numerous as the myomeres.
The ribs support the body walls. They coincide with the myocommata and are probably independent formations, but in some Pisces they are continuous at an early period of development with the haemal arches. They always ossify independently. At their dorsal extremities they articulate with the haemal arch, or the transverse processes, or with the vertebral centrum, as well as with a transverse process. The double articulation in Urodele Amphibia is brought about by the addition to the primitive rib of a dorsal rod which fuses with it and is thus connected to the transverse process of the neural arch. The persistence of this dorsal element, its loss, or the loss of the ventral articulation, is sufficient to explain the variations observable in the way the ribs articulate with the vertebrae in higher Vertebrata. In Amniota the ventral ends of the ribs meet in the middle line. A ventral segment is cut off from each rib, and from the elements thus derived the sternum (costal sternum) is formed. The remainder of the rib is generally divided into a vertebral and a sternal section, with an intercalated 'intermediate rib' in some Reptilia.
1 The ribs extend outwards horizontally in the fibrous septum between the dorso-lateral and ventro-lateral muscles in Elasmobranchii. It is possible that this position is secondarily acquired. In other Vertebrata they lie close to the peritoneum.
All Vertebrata possess typically two pairs of limbs - the pectoral and pelvic, one or the other, or both of which are sometimes aborted. It is still doubtful however if they were ever developed in the ancestors of the Cyclostomi. They appear to have arisen at first as a continuous ridge on each side of the body. A trace of this origin is seen in the ontogeny of Elasmobranchii. The primitive position of the limbs is one of horizontal extension, but it is only incompletely retained in Elasmobranchii and not at all elsewhere. The skeleton of such a primitive limb appears as a basal bar or plate of cartilage, the outer edge of which segments into radially placed radialia. These are borne upon the rest of the bar which divides into two or three large pieces - pro-, meso-, and meta-pterygium, or the two latter only. The radialia are bordered by an integumental fold, supported by fine rays similar to those of the azygos fins. In connection with the horizontal bar of cartilage in each limb is a vertical rod of cartilage possibly derived in the first instance from the horizontal bar. This vertical rod forms the shoulder girdle for the fore-limbs, the pelvic for the hind. The shoulder girdle in Pisces becomes connected in most cases with investing bones.