24 PROF. G. 13. HOWES AND MK. U. H. SWINNERTON ON THE 



Equally significant, as proving that the splitting phenomenon is secondary, is the 

 fact, already pointed out by Gegcnbaur in 18()4, that the clianges 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. lu 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 tlie bony vertebra and ultimately of the 

 chordal plate itself (cf nj)., PI. 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 striatiou radiating from the split (c/. PI. I. fig- 17, ,r.) such as we have 

 encountered nowhere else. The bony vertebrse, 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 (ojj. cit. p. 365) to the fact that in the 

 caudal region the opposite ends of the vertebrse, 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, i.sg.) becomes divided throughout its intervertebral region 

 by a cartilaginous plate {n.]).') lying wholly within the chordal sheath, continuous with the 

 tunica chordae, and having essentially the same structure and evidently the same origin 

 as the chordal plate (ii.jj.). Indeed, the first indications of this are met with at Stage T, 

 as a well-marked thickening of the tunica chordae and its associated cellular tissue. 

 As fully diflferentiated, this, which we propose to term the intervertebral chordal j)late, 

 in contradistinction to the chordal plate which is intravertehral , 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 fiexibility, furnishes additional 

 support. It would thus appear that under the weakening influence associated with the 



