DEVELOPMENT OF THE SKELETON OF THE TUATARA. 49 
differentiation occurring at the base of the extrastapedial (Pl. V. fig. 124), the 
primary independence of which has never been suggested |, it is insignificant indeed. 
Holding the foregoing facts proof sufficient that the columella, extra- and supra- 
stapedial are but the products of specialization of the upper extremity of the hyoid 
arch, with which they are at all stages continuous, we have in conclusion to direct 
attention to some puzzling procartilaginous tracts, of which we have been unable to 
detect a counterpart at a later stage. ‘They are of the nature of two pairs of rounded 
masses (one of which is represented in PI. V. fig. 1,+) disposed serially between the 
roof of the pharynx and the auditory capsule, ventrad of the fenestra ovalis region, and 
we are unable to hazard a guess at their significance. 
Concerning the chondrification of the columella and the related anterior cornu of 
the hyoid, we find at Stage Q, when the procartilaginous tracts above described become 
hyaline, that all traces of the “jointing” disappear. Examination of figs. 6 to 9 
(Pl. V.) shows this conclusively, and also that the hyoid cornu (h.') passes continuously 
into the extrastapedial (ec.), and through that into the columella (st.). The inner 
head of the latter, seen at the procartilaginous stage (Pl. V. figs. 3 & 4, st.) to merge 
into the feebly differentiated mass (¢.aw.) giving rise to the auditory capsule, at the 
period of chondrification is seen to be structurally continuous with that now hyaline, 
and in section (PI. V. fig. 6, st.) delimitable only by a close approximation of its 
peripheral cells. Later, it becomes free, and attached to the rim of the fenestra ovalis 
by membrane, in the customary fashion (¢f. Pl. V. fig. 12, st.). Its rod-like portion 
commences to ossify superficially, in the manner of the parts of the vertebral column 
and the otoccipital bones, 
The Trabecule.—One very conspicuous feature of the afore-described stages in the 
development of the chondrocranium is the non-extension or incorporation of the 
trabecule into the lateral cranial wall. As pointed out (anted, p. 38), these remain 
passive during the development of this. 
Examination of the adult chondrocranium of Sphenodon, apart from a knowledge 
of its development, might well arouse the supposition that its fenestre are due to 
absorption of an originally continuous wall, of the cartilaginous fish type. One of the 
most certain and fascinating of our results is the discovery that the cartilaginous 
cranial bars are all due to continuous-growth processes, that the correlated fenestrae 
are in no way due to absorption, and that the trabeculee play no part in the formation 
of the lateral cranial wall. From first to last these are simple rods, lost by union 
' Kingsley, the latest worker who has dared to deal with the subject of the general morphology of the 
auditory ossicles (with which we are not concerned), expresses his conyiction (Tufts Coll. Studies, No. 6, 1900, 
pp. 215-216) that the Lacertilian hyoid and columella are originally continuous in the embryo. He claims 
for the “ extra columella” an independent origin, but since he uses the term in an unconventional sense and 
his figure (which does not bear out his description) leaves us in doubt whether his ‘“ extra columella” may 
not represent the supra-stapedial process, we feel justified in the above statement. 
VOL. XVI.—ParT 1. No. 7.—february, 1901. H 
