DEVELOPMENT OF THE SIvELETON OE THE TUATARA. 23 



Tertebral 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 vertebrae numbering 6 to 8, and it may be present for all 

 posterior to it. Giinther, 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. PI. I. fig. 18), but we have noted 

 that while the suture may occasionally pass through the expanded base of the trans- 

 verse process ^, 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 vertebrse prove conclusively 

 that although an elongation is eff'ected when the suture is present, it is a subdivision 

 and not a duplication which has taken place, since in vertebrae 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!, PI. I. fig. 18), when examined in situ, are found 

 (fig. 19) to extend into the walls of the haemal canal, to which they act as extra 

 supports, 



' Miiller, H. : Ueber Kegeneration d. Wirbelsaule u. d. Eiiokenmarks b. Triton u. Eidechsen. 4to. Frankfurt, 

 1864. For other references, see Gadow, op. cit. 



^ We consider Cligny in error in his recent argument (" Miscell. Biologiques," Travaux d. 1. Station Zool. 

 d. Wimereux, torn. 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- 

 ventralia " alone is completely opposed to it. 



^ So called by Cope in Dipsosaurus and Sauromalus, Proc. Amer. Phil. Soc. vol. xsx. 1892, pp. 202-205. 

 Comparison with Sphenodon renders it extremely doubtful if this is a product of division, as assumed by 

 Boulenger (P. Z. S. 1391, p. 169). We would rather regard it as a secondarily formed outgrowth like the 

 autogenous hypapophyses which we herein describe. 



