DEVELOPMENT OF THE SKELETON OF THE TUATARA. 19 
accompanying indications of there having been a similar change in the general substance 
of the chorda and the adjacent tip of the odontoid vertebra. 
The neural arches at this stage call for no special comment. ‘Turning to the inter- 
centra, a further suppression of the primary paired series has to be recorded—viz., that 
of the bodies related to segments 10 to 12 and 26 to 29, thus giving a condition of 
the complete absence of these between and including 10 to 29. With careful focussing 
the disappearing primary intercentra (the mode of removal of which we were unable 
earlier to determine) can be at this stage observed to lie wholly within the substance 
of the skeletogenous tissue (cf. 7.p.', Pl. I. figs. 9 & 10); and by comparison of the 
several segments we have convinced ourselves that their disappearance is not due to a 
process of absorption and decay, but to one which may best be described as a sharing 
in the progressive differentiation of the skeletogenous tissues—. é., there is no loss 
of substance, but merely a histological change, leading to a loss of individuality. 
On the completion of this remarkable process, the intercentra are present only for the 
segments 1 to 9 and from the 30th backwards, 7. e., in precisely those regions in which 
in the Lacertilia they are most generally retained; and, as concerning the more precise 
limitations of this intercentrumless area, this comparison becomes more close as the 
development of Sphenodon advances—until a passing condition is reached in which, 
as regards its intercentra, the backbone becomes, as it were, that of a Lacertilian. The 
anterior intercentra remain unossified, but it is during this stage that the caudal ones 
(chevrons) ossify, and that as the result of a superficial deposit, which in its early con- 
dition, as is the case with the neural arches, forms a bony shell (¢f. Pl. II. fig. 13, 2.p.", 
and figs. 4 & 5, na.). Owing to the rapidity with which this process is effected, we 
have not been able to ascertain whether ossification arises at more than a single 
centre. 
Stage T.—The most conspicuous feature of this, the first stage after hatching, is the 
great extension of ossification and calcification, with accompanying constriction of 
the mid-vertebral areas and a general dawning of the adult characters. The process 
of ossification in the centra (ov., Pl. I. fig. 13) has invaded all but the innermost two 
to three rows of cells, which are still but calcified ; and it is densest at the point. of 
greatest constriction. Both within the intervertebral areas (¢.a.) and the central 
substance of the chordal plates (n.p.) calcification has become highly conspicuous, and 
in the case of the latter it extends into the processes afore-mentioned, which .pass into 
the chorda-tissue. 
The cells of the skeletogenous tissue which mark the boundary-lines between the 
intervertebral masses and the vertebree have undergone a complicated rearrangement. 
These lines are delimitable by the fact that the nuclei of their cells, which are small, 
are vertically disposed in close aggregation, as compared with those of the central 
intervertebral series which are larger and run parallel to the longitudinal axis. Those 
D2 
