The ontogenic development of the vertebrae in some gekkonoid lizards.

Abstract

AbstractThe ontogeny of amphicoelous vertebrae was studied in Ptyodactylus hasselquistii and Hemidactylus turcicus, and that of procoelous vertebrae, in Sphaerodactylus argus. The embryos were assigned arbitrary stages, drawn to scale, and mostly studied in serial sections.Resegmentation occurs as in all amniotes. A sclerocoel divides each sclerotome into an anterior “presclerotomite” and a denser posterior “postsclerotomite.” Tissue surrounding the intersegmental boundary forms the centrum, which is intersegmental. Tissue around the sclerocoel builds the intervertebral structures, which are midsegmental. In the trunk and neck, postsclerotomites form neural arches, and presclerotomites build zygapophyses.The adult centrum consists of the perichordal primary centrum, plus neural arch bases (= secondary centrum). Between the latter and the arch proper, a neurocentral suture persists until obliterated in maturity. A dorso‐ventral central canal persists on either side of the primary centrum, between the latter and the secondary centrum.The notochord becomes true cartilage midvertebrally in all vertebrae, and elastic cartilage intervertebrally in the posterior caudal region. Elsewhere its characteristic tissue persists.Intervertebrally, cervical hypapophyses, caudal chevrons and chevron‐bases in the trunk are preformed early in cartilage. Directly ossifying median intercentra are added later in all regions.The first cervical presclerotomite is absent: the hypapophysis (= corpus) of the atlas consists exclusively of postsclerotomitic tissue, there is no proatlas, and the odontoid lacks the apical half‐centrum present in other lepidosaurians.In the autotomous caudal region presclerotomites are as prominent as postsclerotomites. Both build neural arches, the two arches of each vertebra remaining distinct and ossifying separately, so that the intersegmental autotomy split persists between them.The last sclerotome is complete, its postsclerotomite forming a half centrum which ossifies.In Sphaerodactylus, while the vertebrae ossify, each intervertebral ring becomes concave anteriorly, convex posteriorly; it remains as a cushion between the condyle and a facet formed by differential growth of the centra. Thus these procoelous centra resemble the amphicoelous centra of Ptyodactylus and Hemidactylus, rather than the procoelus centra of other squamates.The vertebral column of Gekkonoidea closely resembles in its development and microscopical structure that of Sphenodon.