Of these various interpretations, that of Naef seems to involve the minimum of novelty, namely, that the rib-bearer is the basiventral, expanded and external to the vertebral artery. It is not necessary to take this modification as the ancestral condition in tetrapods, of course. The basiventral (=intercentrum) would merely have expanded sufficiently to provide a diapophysis for the tuberculum as well as the (primitive) facet for the capitulum. No neomorph appears under this hypothesis, which has the distinct advantage of simplicity. Figures of early stages in vertebral development by the authors mentioned show that the basidorsals chondrify first, as neural arches, while a separate mass of mesenchyme lies externally and ventrally from these. This mesenchyme may chondrify either in one piece (on each side) or in two; in _Molge_ the part adjacent to the centrum is ossified in the 20-mm. larva, and subsequently unites with the more dorsal and lateral cartilaginous part, while the rib, appearing farther out, grows inward to meet this composite "rib-bearer." In _Necturus_ the mesenchyme below the neural arch differentiates into a cartilage below the vertebral artery (position proper to a basiventral), a bridge between this and the neural arch, and a rib, the latter two chondrifying later than the "basiventral" proper. In the "axolotl" (presumably _Ambystoma tigrinum_) the rib-bearer grows downward from its first center of chondrification at the side of the neural arch (Emelianov, 1936). Thus it appears that the simplest hypothesis to account for the rib-bearer is that (_a_) it is the basiventral, (_b_) it is recognizable just before chondrification as a mass of mesenchyme in contact with both the notochordal sheath and the basidorsal cartilage, (_c_) it may chondrify or ossify first in its ventral portion or in its dorsal portion, the two then joining before it fuses with the rest of the vertebra, (_d_) the enclosure of the vertebral artery is a consequence of the extension of the basiventral beyond the position occupied by it in primitive Amphibia, and (_e_) there is no indication that this took place in other orders than the Urodela. It seems that the vertebrae in Urodela have at least the following components: perichordal centra, separate basidorsal cartilages, and basiventrals, which are somewhat specialized in their manner of development. The vertebrae of Anura develop in the fashion just described except that basiventrals are lacking. It would seem no more difficult to accept the derivation of salamander vertebrae from the temnospondylous type than it is in the case of frogs, if other evidence points to such an ancestry.