DEVELOPMENT OF THE SKELETON OF THE TUATARA. 07 



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 intcrvertebrai 

 regions and the intercentra the tubercular with the areas of differentiation of the 

 vertebral centra and arches of the vertebrae behind. That these relationships are lost 

 during later development for all but the third and fourth praesternal segments. 



8. That the uncinates are separate in origin, with the possible exception of those of 

 the penultimate and antepenultimate praesternal 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 fenestrse are all attributable to 

 direct processes of growth i. <?., 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 trabeculae cranii play no part in the direct formation of the lateral 

 cranial wall, and that they represent a pair of prseoral visceral arches. 



K2 



