The development of the vertebrae proceeds as follows. First appear a pair of basidorsalia and a pair of basiventralia (Fig. 1, 1, B.D, B.V), blocks of cartilage, imbedded in and resting upon the thin sheath of the chorda dorsalis. Next appears a pair of interdorsal blocks, immediately behind the basidorsals; and somewhat later appears a pair of interventral blocks. These four pairs of cartilages or "arcualia" each meet, above or below the chorda, and form semi-rings, which again by extending upwards or downwards fuse into complete rings, in such a way that the interdorsal and interventral elements form the intervertebral mass spoken of above. The basidorsals fuse with the basiventrals, and form the body of the vertebra, the fusion being effected chiefly by the calcification and ossification of the lateral connecting portion of the skeletogenous layer. The basidorsalia form the neural arches with their unpaired short spinous or neural, and the paired anterior and posterior zygapophysial processes. Concerning the {13}basiventralia we have to distinguish between the trunk and the tail. In the latter they produce a pair of ventral outgrowths or haemapophyses, which ultimately enclose the caudal blood-vessels. In the trunk the basiventral blocks of cartilage are suppressed; they appear in the early larvae, but disappear during or even before metamorphosis. [Illustration: FIG. 1.–1-5, Five successive stages of the development of a caudal vertebra of a newt; 6-7, the second and the first cervical vertebra of _Cryptobranchus_; 8-9, side view of the constituent cartilaginous blocks of a caudal vertebra (8) and a trunk-vertebra (9) of _Archegosaurus_ as typical examples of Temnospondylous quadripartite and tripartite vertebrae. The cross-hatched parts indicate the articular facets for the ribs. The anterior end of all the vertebrae looks towards the right side. _af_, In 7, articulating facet for the occipital condyle; _B.D_, basidorsal piece or neural arch; _B.V_, basiventral piece or ventral arch; _Ch_, chorda dorsalis, or notochord; _I.D_, interdorsal piece; _I.V_, interventral piece; _I.V.L_, intervertebral ligament; _N_, spinal nerve–these are numbered I, II, III in 6 and 7; _R_, rib; _T_, in 7, rib-like tubercle on the first vertebra.] Towards the end of the tail the vertebrae diminish in size, and their constituent cartilages assume a more and more indifferent shape, until they become confluent into a continuous rod of cartilage, resembling in this respect the Dipnoi and Holocephali. A periodical revival of this rod, at least of its connective tissue, appears in the tail-filament of the male _Triton palmatus_ during the breeding-season. The first vertebra, called the atlas, because it carries the head, is remarkable for the possession of an odontoid process. The latter is formed by a pair of cartilages and represents part of a vertebra, the dorsal portion of which seems to have been added to the occipital part of the cranium.