In the tetrapods the conus develops similar endocardiac ridges to those seen in lungfish, but the conus has now greatly shrunken in length, with the result, owing to its ends being fixed, that the Z-flexure is drawn out and replaced by a spiral twist. Further, in the Amniota, the two prominent ridges, which in the lungfish merely overlap, undergo complete fusion, so that the cavity of the conus, as of the ventral aorta, becomes divided completely into pulmonary and systemic cavities, continuous respectively with the right and left ventricles, which also become completely sepa rated during development. In the higher Amniota the septum so formed in the conus becomes itself split, so that the conus comes to be represented by two separate vessels, pulmonary and sys temic, spirally twisted round one another.
Of all features in the development of the blood system of vertebrates, perhaps the most interesting is that the great arteries of the higher amniotes repeat in the course of their development the series of aortic arches between the visceral clefts. Although the bird or human being will never use its gill-clefts for breathing, yet it shows for a time the typical piscine arrangement of aortic arches. As development proceeds, large tracts of this primitive scheme disappear while others persist and become straightened out into the great arteries of the adult.
The venous system of the vertebrate shows also many features of interest in its embryology. Perhaps the most important of these is that the venous system of the higher vertebrates shows i for a time in the embryo the same main trunks—duct of Cuvier, anterior and posterior cardinal veins—as those of an adult fish. The main new development in the venous system of tetrapods, the inferior or posterior vena cava, presents the striking pecu liarity that it has a double origin in the embryo, its anterior por tion being associated with the liver and its posterior portion with the posterior cardinal veins. This points to the posterior vena cava having originated in evolution from an arrangement similar to that of modern dipnoans, where the anterior end of the opisthonephros is fused with the tip of the liver, thus rendering possible the direct passage of blood from kidneys to heart through the liver substance.
skeletal development is the modification of patches of connective tissue to form cartilage. In the trunk region these appear first as the paired rudiments of neural and haemal arches, two pairs of each in the lower types within the length of a single muscle seg ment. In two of the more archaic groups, Elasmobranchii and Dipnoi, cartilage cells from the arch-rudiment burrow through the primary sheath and colonize the secondary sheath of the notochord. The notochord thus becomes enclosed in a cylinder of cartilage, which in the elasmobranch becomes segmented into vertebral centra, each carrying two pairs of neural and of haemal arches. In other vertebrates this invasion of the secondary sheath by cartilage cells does not take place, and the centra arise outside the primary sheath by expansion of the bases of the arches. In the head region traces of cartilaginous vertebrae can be traced in various archaic vertebrates as far forwards as the tip of the infundibulum, the hinder region of the cranium representing a part of the axial skeleton in which the vertebral segmentation has disappeared and the neural canal become greatly enlarged in correlation with the expansion of the central nervous system to form the brain. As regards the pre-chordal or trabecular portion of the cranium, embryology does not provide any definite evidence as to its relation to the trunk skeleton. The olfactory organ and the otocyst each becomes enclosed in a capsule of cartilage and these become incorporated in the complete cranium. Apart from the axial skeleton cartilaginous elements make their appearance in relation to the visceral arches (see VERTEBRATA) and to the limbs, each showing in their development many details of interest. The bony skeleton makes its first appearance in the isolated placoid scales of elasmobranchs, some of which, it will be remem bered, are carried into the buccal cavity, either remaining of small size or becoming enlarged to form the teeth. In various of the lower vertebrates, especially urodele amphibians, embryology dem onstrates that the bones which underly the base of the cartilagi nous cranium and reinforce it are formed of originally separate placoid denticles, which become united into a continuous plate by the spreading out and fusion of their basal portions. In some cases only part of the bone may show this dental origin, the den ticles having disappeared from the rest. In other cases bones which in Urodeles have this dental origin develop in Anura with out showing denticles. Such and many similar facts of embry ology have suggested the working hypothesis that the bony skele ton had its evolutionary origin in the primitive dermal equipment of placoid scales.