DEVELOPMENT OF THE SKELETON OF THE TUATARA. 21 
fibrous, and on the ventral side they, together with the subvertebral fibrous tissue (f.t.), 
have attained to full differentiation. Their cells are arranged in parallel series, with 
an accompanying loss of individuality, the tissues having the appearance of being 
permeated by widely-separated parallel rows of small and elongated nuclei. While 
peripherally this parallel arrangement becomes more conspicuous, in the deeper layers 
the fibres become stronger, their attachments to the bony vertebre appearing more 
marked, as the result of their greater avidity for the stain. Owing to the increase in 
thickness of the bony shell of the vertebral body (still largely composed throughout 
its inner moiety of calcified cartilage), the chordal plates (Pl. I. fig. 15 and text-fig. 4, 
n.p.) are comparatively insignificant as compared with the earlier stages. They are, 
however, present, and, owing to their completely septate nature, the chorda is now 
broken up into a series of elongated and recurrent interseptal segments, each of which 
is seen to be bounded not by the chordal epithelium hitherto recognizable, but by a 
deeply-staining and structureless cuticle, which passes into the faces of the plates, the 
superficial portions of which stain correspondingly with it. 
Fig. 4. 
Median longitudinal section of the adult vertebral column (caudal region) of Sphenodon, to show the tunica 
chorde (t.c.) and its relationships to the chordal sheath and plate. Camera lucida, x 70. 
i.sq., interseptal segment of chorda; n.c.s., chordal sheath; x.p., chordal plate; 0.v., osseous vertebra ; 
t.c. tunica chorde. 
This cuticle, which we propose to term the tunica chordw (text-fig. 4, t.c.), would 
appear to represent the chordal epithelium ; but that it underlies and is independent 
of the chordal sheath (nc.s.) is proved by the fact that this stains lightly and can 
