DEVELOPMENT OF THE SKELETON OF THE TUATAEA. 25 



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 Dohvn 

 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 us 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, i.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. pi. xvii. fig. 25, a;). 

 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 diflFerentiates 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 wliich 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., PI. I. fig. 16), we have noted that for segments 12, 13, 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 prsesternal intercentra of L. agilis {op. cit. pi. 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. torn. ii. 18S3, p. 324. 



VOL. XVI. — PAKT T. No. 4. — Fehruanj, 1901. e 



