DEVELOPMENT OF THE SKELETON OF THE TUATARA. 20 
splitting of the strongest part of the vertebral complex, a compensating development is 
deliberately initiated in the intervertebral—a process to which the term of Dohrm 
and Kleinenberg, “‘ substitution of organs,” may be not inappropriately applied; and 
beyond the extent to which these intervertebral plates may furnish support and be 
concerned in determining points of muscular attachment, we are inclined to regard the 
intravertebral plates as direct agents in the formation of the axis of the reproduced 
tail. This matter, however, requires further investigation. There remain only for 
consideration the definitive intercentra of the region of disappearance of those of 
the primary series, viz. segments 5(?)-6 to 30. The intervertebral region of each of 
these shows an intercentrum of considerable proportions (PI. I. fig. 15, ¢.s.). This, the 
secondary intercentrum, agrees with those already described (antea, p. 20) in lying 
outside the skeletogenous tissues, as figured by Goette (op. cit. pl. xvii. fig. 25, x). 
Not wholly so, however! ‘The most distinctive character of these remarkable bodies is 
a sharp demarcation into a greater feebly-staining outer portion and a lesser inner one, 
which differentiates darkly and, when viewed in section, tapers into the intervertebral 
mass. The cells of their inner moiety are more numerous and much larger than those 
of their outer; and since in the mode of disposition these are linear with those of 
the intervertebral mass, they would appear to be derived from it. If this be so, these 
definitive secondary intercentra would appear to represent those to which we originally 
applied the term, but with a superaddition of parts derived from the intervertebral 
tissues. Be this as it may, their formation finally completes the series of inter- 
centra, which are now present throughout the whole length of the column. 
Typically these secondary intercentra arise singly, as median and transverse masses. 
In one case, however (i.s., Pl. J. fig. 16), we have noted that for segments 12, 15, and 
16 to 20 (7 in all) they are paired. Boulenger has drawn attention (P. Z.S. 1891, p. 170) 
to the existence in Lacerta ocellata of paired intercentra in the posterior sacral and 
anterior caudal regions, and Leydig has figured a similarly-paired condition of one of 
the preesternal intercentra of L. agilis (op. cit. pl. iv. fig. 53). In the absence of any 
knowledge of secondary intercentra in the Lacertilia, we conclude that while in both 
species the paired elements are of primary order, in our exceptional Sphenodon 
those of the post-sternal region, which are paired, would seem to be of secondary, 
and that these may therefore be variable as to their median or paired nature. 
Comparison of the caudal region of Sphenodon with that of Lacerta ocellata as 
described by Boulenger would seem to justify the conclusion that in the chevrons 
primary intercentra are alone represented. We find, however, that in the case of the 
first 4-5 this is not wholly so. In the adult Sphenodon, these are characterized by the 
fact that their proximal ends are united, as pointed out by Dollo in 1883, whereby 
they appear to overarch the caudal canal. Credner has confirmed this observation, and 
‘ Dollo: Bull. Mus. Hist. Nat. Belg. tom. ii. 1883, p. 324. 
VoL. XvL—Ppart 1. No. 4.—February, 1901. E 
