Fig. 239. - Embryology of Vertebrata. A, Portion of the germinal area of the ovum of a Bitch, showing the primitive groove (after Bischoff). B, Profile view of the same. C, Diagram representing the amnion and allantois : e Embryo ; a Amnion ; u Umbilical vesicle; b Allantois; f Pedicle of the allantois, afterwards the urinary bladder. D, Head of an embryo, showing the visceral arches (v v).
Furthermore, the floor of the "primitive groove" in the embryo of all Vertebrates has developed in it at an early-period the structure known as the "notochord" or " chorda dorsalis" (fig. 238, B, ch). This structure, doubtfully represented in any Invertebrate, is a semi-gelatinous or cartilaginous collection of cells, forming a rod-like axis, which tapers at both ends, and extends along the floor of the cerebro-spinal canal, supporting the cerebro-spinal nervous centres. In some Vertebrates, such as the Lancelet (Amphioxus), the notochord is persistent throughout life. In the majority of cases, however, the notochord is replaced before maturity by the structure known as the "vertebral column" or "backbone," from which the sub-kingdom Vertebrata originally derived its name. This is not the place for an anatomical description of the spinal column, and it is sufficient to state here that it is essentially composed of a series of cartilaginous, or more or less completely ossified, segments or vertebra, arranged so as to form a longitudinal axis, which protects the great masses of the nervous system. It is to be remembered, however, that all Vertebrate animals do not possess a vertebral column. They all possess a notochord; but this may be persistent, and in many cases the development of the spinal column is extremely imperfect.
Another embryonic structure which is characteristic of all Vertebrates, is found in the so-called " visceral arches" and "clefts" (fig. 239, D). The "visceral arches" are a series of parallel ridges running transversely to the axis of the body, situated at the sides of, and posterior to, the mouth. As development proceeds, the intervals between these ridges become grooved by depressions which gradually deepen, until they become converted into a series of openings or "clefts," whereby a free communication is established between the upper part of the alimentary canal (pharynx) and the external medium. In Fishes and many Amphibians the greater number of the visceral clefts remain open throughout life; and the visceral arches of all fishes (except the Lancelet) throw out filamentous or lamellar processes, which receive branches of the aorta and constitute branchiae. In the higher Vertebrata all the visceral clefts become closed, whilst no branchiae are ever developed upon the visceral arches.
The limbs of Vertebrate animals are always articulated to the body, and they are always turned away from the neural aspect of the body. They may be altogether wanting, or they may be partially undeveloped; but there are never more than two pairs, and they always have an internal skeleton for the attachment of the muscles of the limb.
A specialised blood-vascular or "haemal" system is present in all the Vertebrata ; and in all except one - the Amphioxus - there is a contractile cavity or heart, which never consists of less than two chambers provided with valvular apertures. In all the Vertebrata the heart is essentially a respiratory heart - that is to say, it is concerned with driving the impure or venous blood to the breathing organs; and in its simplest form (fishes) it is nothing more than this. In the higher Vertebrates, however, there is superadded to this a pair of cavities which are concerned in driving the pure or arterial blood to the body. In the case of the Mammals, these two circulations are often spoken of as the "lesser" or "pulmonary" circulation, and the "greater" or "systemic" circulation.
In all Vertebrates there is that peculiar modification of the venous system which is known as the "hepatic portal system." That is to say, a portion of the blood which is sent to the alimentary canal, instead of returning to the heart by the ordinary veins, is carried to the liver by a special vessel - the vena portae - which ramifies through this organ after the manner of an artery.
In all Vertebrates, also, is found the peculiar system of vessels known as the " lacteal system." This is to be regarded as an appendage of the venous system of blood vessels, and consists of a series of vessels which take up the products of digestion from the alimentary canal, elaborate them, and finally empty their contents into the veins.
Lastly, the masticatory organs of Vertebrates are modified portions of the walls of the head, and never " hard productions of the alimentary mucous membrane, or modified limbs " (Huxley), as they are amongst the Invertebrata.
The above are the leading characters of the Vertebrata as a whole; but before going on to consider the primary divisions of the sub-kingdom, it may be as well to give a very brief and general description of the anatomy of the higher and more typical Vertebrates, commencing with their bony framework, or skeleton.
The skeleton of the Vertebrata may be regarded as consisting essentially of the bones which go to form the head and trunk on the one hand (sometimes called the " axial" skeleton), and of those which form the supports for the limbs (" appendicular " skeleton) on the other hand. The bones of the head and trunk may be looked upon as essentially composed of a series of bony rings or segments, arranged longitudinally, one behind the other. Anteriorly these segments are much expanded, and likewise much modified, to form the bony case which encloses the brain, and which is termed the cranium or skull. Behind the head the segments enclose a much smaller cavity, which is called the "neural" or spinal canal, as it encloses the spinal cord; and they are arranged one behind the other, forming the vertebral column. The segments which form the vertebral column are called "vertebrae," and they have the following general structure : Each vertebra (fig. 240, A) consists of a central piece, which is the fundamental and essential element of the vertebra, and is known as the "body" or "centrum" (V). From the upper or posterior surface of the centrum spring two bony arches (n n), which are called the "neural arches" or "neurapophyses," because they form with the body a canal - the "neural canal" - which encloses the spinal cord. From the point where the neural arches meet behind, there is usually developed a longer or shorter spine, which is termed the " spinous process," or "neural spine" (s). From the neural arches there are also developed in the typical vertebra two processes (a a), which are known as the "articular " processes, or "zyga-pophyses." The vertebrae are united to one another partly by these, but to a greater extent by the bodies or "centra." From the sides of the vertebral body, at the point of junction with the neural arches, there proceed two lateral processes (d d), which are known as the "transverse processes." (In the typical vertebra the transverse processes consist each of two pieces, an anterior piece or "parapophysis," and a posterior piece or "diapophysis.") These elements form the vertebra of the human anatomist, but the "vertebra" of the transcendental anatomist is completed by a second arch which is placed beneath the body of the vertebra, and which is called the "haemal" arch, as it includes and protects the main organs of the circulation. This second arch is often only recognisable with great difficulty, as its parts are generally much modified, but a good example may be obtained in the human chest, or in the caudal vertebra of a bony fish.