DEVELOPMENT OF THE SKELETON OF THE TUATARA, 67 
8. SUMMARY AND CONCLUSIONS. 
Summary.—The following is a summary of the more important conclusions embodied 
in Sections 5 to 7 of the present Memoir :— 
1. That 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. 
2. That 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. 
3. That the hyaline cartilages which go to form the vertebral centra are paired in 
origin. 
4, That an intravertebral chordal plate is formed by transformation of the sub- 
stance 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. 
5. That during the final differentiation of the vertebrae the chorda becomes 
metamerically segmented, and that there appears in each segment a central chordal 
vesicle at the point of greatest flexibility. 
6. That 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. 
7. That the ribs arise in procartilage, in relation to the primary intercentra, with 
which they are the first differentiated skeletal elements; and that they are differen- 
tiated 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 vertebree behind. That these relationships are lost 
during later development for all but the third and fourth presternal segments. 
8. That the uncinates are separate in origin, with the possible exception of those of 
the penultimate and antepenultimate preesternal ribs. 
9. That the cartilaginous brain-case is a product of the union of distinct ethmo- 
and orbito-sphenoidal plates, and that its bars and fenestre are all attributable to 
direct processes of growth—/. e., that it is not a fenestrated cranium of the cartilaginous 
fish type. 
10. That 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. 
11. That the trabecule cranii play no part in the direct formation of the lateral 
cranial wall, and that they represent a pair of preoral visceral arches, 
K2 
