DEVELOPMENT OF THE SKELETON OF THE TUATARA, 23 
vertebral axis (apart from outstanding processes and chevrons) lie in the study of the 
“splitting” phenomenon and the intervertebral chorda. Since the days of Cuvier, 
a good deal of interest has centred in the study of this process, by which the 
Lacertilian tail may be cast off. Hyrtl, Gegenbaur, Miiller!, and Leydig have more 
especially studied its details, which involve to a conspicuous degree the chordal plate, 
as Gegenbaur first pointed out. Gadow has recently summarized our knowledge of 
the nature and extent of the subdivision-line recognizable on the exterior of the 
vertebra capable of “ splitting” among certain Lizards ; and perusal of his statements 
will show that as to its position and relationships to the neural arch and transverse 
processes variability may be recognized, the “split” subdividing the vertebra in some 
cases into equal, in others into unequal portions. In Sphenodon this suture usually 
first appears on the caudal vertebra numbering 6 to 8, and it may be present for all 
posterior to it. Gtinther, who first described it in Sphenodon, states (67. p. 606) 
that it ‘‘ passes through the middle and behind the transverse process,” Gadow, on 
the other hand, figures it as passing in front and subdividing the vertebra into 
approximately equal halves, of which the posterior bears both transverse processes and 
neural spine. With this we are in agreement (cf. Pl. I. fig. 18), but we have noted 
that while the suture may occasionally pass through the expanded base of the trans- 
verse process 7, it may also subdivide the neural spine (as in PI. I. fig. 20) in a manner 
anticipatory of the more symmetrical division which, according to Gadow (p. 29, 
figs. 22 and 23), appears to involve the anterior neural spine® of Lacerta ocellata. 
And further, the characters of the first three suture-bearing vertebree prove conclusively 
that although an elongation is effected when the suture is present, it is a subdivision 
and not a duplication which has taken place, since in vertebre which are in all other 
respects identical with those immediately in front, the suture, which is restricted to 
the centrum in nos. 9 and 10, passes gradually up to the arch, assuming the condition 
characteristic of those behind. 
We have noted that in the adult vertebral column both halves of the splitting 
vertebra send down minute but definite autogenous hypophyses, which have not before 
been described. ‘These (ha.', Pl. I. fig. 18), when examined in situ, are found 
(fig. 19) to extend into the walls of the hemal canal, to which they act as extra 
supports. 
1 Miller, H.: Ueber Regeneration d. Wirbelsiiule u. d. Riickenmarks b. Triton u. Kidechsen. 4to. Frankfurt, 
1864. For other references, see Gadow, op. cit. 
2 We consider Cligny in error in his recent argument (‘‘ Miscell. Biologiques,” Travaux d. 1. Station Zool. 
d. Wimereux, tom. vii. 1899, p. 432), that the power of fracture and the “ double pleurapophysis ” are associated 
with ancestral characters. The probability that the centra of the Amniota are compounded of the “ intra- 
yentralia ” alone is completely opposed to it. 
2 So called by Cope in Dipsosaurus and Sauromalus, Proc. Amer. Phil. Soc. vol. xxx. 1892, pp. 202-205. 
Comparison with Sphenodon renders it extremely doubtful if this is a product of division, as assumed by 
Boulenger (P. Z. 8. 1891, p. 169). We would rather regard it as a secondarily formed outgrowth like the 
autogenous hypapophyses which we herein describe. 
