The centrum in Anura (Fig. 5) is formed in the perichordal sheath (_Rana_, _Bufo_) or only in the dorsal portion thereof (_Bombinator_, _Xenopus_). The neural arch develops from the basidorsal cartilages that rest upon, and at first are entirely distinct from, the perichordal sheath. Ribs, present as separate cartilages associated with the 2nd, 3rd and 4th vertebrae in the larvae of _Xenopus_ and _Bombinator_, fuse with lateral processes (diapophyses) of the neural arches at metamorphosis, but in _Leiopelma_ and _Ascaphus_ the ribs remain freely articulated in the adult. Basiventral arcualia have been supposed to be represented by the hypochord, a median rod of cartilage beneath the shrinking notochord in the postsacral region, which at metamorphosis ossifies to produce the bulk of the urostyle. Fig. 5, lower right, a transverse section taken immediately posterior to the sacral ribs in a transforming specimen of _Ascaphus_, shows that the "hypochord" is a mass of cartilage formed in the perichordal sheath itself, and very obviously is derived from the ventral part of postsacral perichordal centra; there are, then, no basiventral arcualia, and the discrete hypochord shown in MacBride's diagram (Fig. 5, upper right) of a frog vertebra does not actually occur below the centrum, but only below the notochord in the postsacral region. [Illustration: Fig. 6. Development of Urodele vertebrae. Upper figures, _Triton_: at left, larva at 20 mm., at right, diagram of components of vertebra (from MacBride, 1932, figs. 17, 47C). Middle figures, _Molge vulgaris_ larva: left, at 18 mm.; middle, at 20-22 mm.; right, at 25 mm. (from Emelianov, 1936, figs. 33, 36, 38 respectively). Lower figures, _Necturus maculosus_ larva: left, at 21 mm.; right, at 20 mm. (from MacBride, 1932, figs. 41.5, 41.3 respectively, after Gamble, 1922). All × 20 approx. For explanation of abbreviations see Fig. 3.] In Urodela (Fig. 6) the pattern of vertebral and rib development is more complex, and there has been much controversy over its interpretation. Neural arches and perichordal centra form in the same manner as in frogs, but with the addition in certain cases (_Triton_) of a median supradorsal cartilage, which gives rise to the zygapophyses of each neural arch. Difficulty comes, however, in understanding the relationship of the ribs to the vertebrae. Each rib, usually two-headed, articulates with a "transverse process" that in its early development seems to be separate from both the vertebra and the rib, and is therefore known, noncommittally, as "rib-bearer." This lies laterally from the centrum, neural arch, and vertebral artery; upon fusing with the vertebra it therefore encloses the artery in a foramen separate from the one between the capitulum and tuberculum of the rib (the usual location of the vertebral artery). At least four different interpretations of these structures have been suggested: