24 PROF. G. B. HOWES AND MR. H. H. SWINNERTON ON THE 
Equally significant, as proving that the splitting phenomenon is secondary, is the 
fact, already pointed out by Gegenbaur in 1864, that the changes which render it 
possible appear as, by ossification, the vertebra is assuming the adult condition; and in 
support of this we are able to state that the bony centrum and chordal plate (both of 
which become split as the adult condition is realized) are in the developmental stages 
complete. The first appearance of subdivision is at Stage 'T. In fortunate sections of 
the adult we have been able to determine the probable method of splitting, and that it 
is, at least to a large extent, due to invasion of the bony vertebra and ultimately of the 
chordal plate itself (cf. np., Pl. I. fig. 17) by the surrounding tissues, with accompanying 
absorption, since in sections in which the process was going on we find an inward 
extension of these tissues into the parts already ossified, with an accompanying peri- 
vertebral striation radiating from the split (cf. Pl. I. fig. 17,2.) such as we have 
encountered nowhere else. The bony vertebrae, however, continue to thicken after the 
first appearance of the split, and nowhere more so than at its opposite edges ; wherefore 
it follows that by this, which is a growth process, the split is extended outwards. 
We find in the intervertebral regions of the tail that a series of changes of a some- 
what remarkable and intensely interesting order are undergone. Goette has recently 
described them in part. He calls attention (op. cit. p. 365) to the fact that in the 
caudal region the opposite ends of the vertebra, which in the trunk are usually directed 
outwards, are inwardly thickened, and that they so constrict the vertebral canal that it 
is most spacious within the vertebra and narrowest at its extremities. In this respect 
the caudal vertebra presents a striking contrast with that of the trunk-region, in which 
the ends are widely open and the centre is contracted. He asserts that correlatively 
with this constriction an ingrowth of the intervertebral mass, already foreshadowed in 
the trunk-region, has effected a complete blocking of the intervertebral area, with 
accompanying segmentation of the chorda. When first examined, the intervertebral 
regions of the tail of a mature Sphenodon do appear to be compact, as Goette asserts ; 
but when further investigated in well-preserved material the parts there present are 
seen to be complex, and to bear to one another a relationship essentially similar to that 
which the chordal plate bears intracentrally to the vertebral body. The interseptal 
chordal segment (PI. I. fig. 17, 7.sg.) becomes divided throughout its intervertebral region 
by a cartilaginous plate (w.p.') lying wholly within the chordal sheath, continuous with the 
tunica chorde, and having essentially the same structure and evidently the same origin 
as the chordal plate (n.p.). Indeed, the first indications of this are met with at Stage T, 
as a well-marked thickening of the tunica chorde and its associated cellular tissue. 
As fully differentiated, this, which we propose to term the intervertebral chordal plate, 
in contradistinction to the chordal plate which is intravertebral, reveals terminal calci- 
fications, and a centrally less dense area at the point where movement of the parts 
is greatest, which, while providing for the necessary flexibility, furnishes additional 
support. It would thus appear that under the weakening influence associated with the 
