On the Development of the Skeleton of the Tuatara, Sphenodon punctatus; with Remarks on the Egg, on the Hatching, and on the Hatched Young.

Authors


Abstract

Summary.— The following is a summary of the more important conclusions embodied in Sections 5 to 7 of the present Memoir:—

  • 1That two kinds of intercentra are formed: primary intercentra, originally paired and mostly preformed in cartilage; and secondary intercentra, which replace those throughout certain regions of the body, and are mostly median and arise by direct ossification outside the vertebral column.
  • 2That the primary intercentra persist in the caudal region to form the chevrons, and anteriorly for the first few segments, and that the secondary intercentra coexist with them in the anterior caudal region alone.
  • 3That the hyaline cartilages which go to form the vertebral centra are paired in origin.
  • 4That an intravertebral chordal plate is formed by transformation of the substance of the chorda within each vertebral centrum, and that the plate is continuous with the chordal epithelium, which becomes converted into a tunica lying inside the chordal sheath.
  • 5That during the final differentiation of the vertebræ the chorda becomes metamerically segmented, and that there appears in each segment a central chordal vesicle at the point of greatest flexibility.
  • 6That in that part of the caudal region modified for “splitting” there are formed, by similar differentiation to that which gives rise to the intravertebral chordal plates, a series of intervertebral plates, and that the intravertebral plates play an important part in the casting of the tail, and may be possibly the seat of regenerative activity in the formation of the renewed caudal axis.
  • 7That the ribs arise in procartilage, in relation to the primary intercentra, with which they are the first differentiated skeletal elements; and that they are differentiated obliquely, the capitular portions in direct relationship with the intervertebral regions and the intercentra-the tubercular with the areas of differentiation of the vertebral centra and arches of the vertebræ behind. That these relationships are lost during later development for all but the third and fourth præsterual segments.
  • 8That the uncinates are separate in origin, with the possible exception of those of the penultimate and antepenultimate præsternal ribs.
  • 9That the cartilaginous brain-case is a product of the union of distinct ethmo- and orbito-sphenoidal plates, and that its bars and fenestræ are all attributable to direct processes of growth-i. e., that it is not a fenestrated cranium of the cartilaginous fish type.
  • 10That the hypoglossus nerve-bearing region bears five pairs of nerves with four pairs of foramina, and that during development these become reduced to two each.
  • 11That the trabeculæ cranii play no part in the direct formation of the lateral cranial wall, and that they represent a pair of præoral visceral arches.
  • 12That the epipterygoid bone is the product of ossification of the ascending process of the pterygo-quadrate cartilage, and that the special feature of the quadrate bone is an extension of its ossific head into the latter for approximation to the epipterygoid.
  • 13That the columella auris and stapedial processes are alike direct derivatives of the hyoid arch, with which they are continuous at all stages, and that structural complication is due to overgrowth of the same.
  • 14That the upper end of the hyoid cornu is attached to the quadrate only, and that there is a fleeting “jointing” of the former throughout its course.
  • 15That in the meeting of the pterygoids and vomers, the latter play a part leading up to the conditions occurring in the Chelonia and Plesiosauria.
  • 16The cartilaginous pterygo-quadrate is in its detailed characters most nearly comparable to that of the Apodal Batrachian Ichthyophis.
  • 17That each of the three segments of each of the “abdominal ribs” (gastralia) arises by union of a number of calcifications, and that the median segment may be paired. That the “plastron” is present in a degenerating form.
  • 18That in their early differentiation the membrane-bones of the head and face are predominantly circumorbital in position.
  • 19That there is no supratemporal bone recognizable at any stage as a distinct element.
  • 20That the hip-girdle in its development is of a lower type than the Lacertilian, and that there are two types of pelvis present in adults.
  • 21That the fifth tarsale has no existence in ontogeny, and that the basal element of the fifth digit of the hind limb is a metatarsal.
  • 22That there are apparently three centralia carpi represented during development, and that a centrale is incorporated in the “astragalus.”
  • 23That calcified vomerine teeth are not present during encapsulation within the egg-shell.
  • 24That of the three pairs of upper incisors, the middle certainly disappear and the inner may become tricuspid.
  • 25That the cheek-teeth appear to consist of two series-an early developed alternating set, and a later posterior and structurally uniform set.
  • 26That there arise within the occipital region and the individual vertebral segments sustentacular ligaments, for support of the medulla and spinal cord.

Ancillary