DEVELOPMENT OF THE SKELETON OF THE TUATAEA. 49' 



diiFerentiation occurring at the base of the extrastapeclial (PL V. fig, 12 5), the 

 primary independence of which has never been suggested ^, it is insignificant indeed. 



Holding the foregoing facts proof suflficient 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. l,t) 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 

 (PI. V.) shows this conclusively, and also that the hyoid cornu (/*.') passes continuously 

 into the extrastapedial (ec), and through that into the columella (st.). The inner 

 head of the latter, seen at the procartilaginous stage (PI. V. figs, 3 & 4, st.) to merge 

 into the feebly diff'erentiated mass [c.au.) 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 [cf. PI. 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 Trctbeculm. — One very conspicuous feature of the afore-described stages in the 

 development of the chondrocranium is the non-extension or incorporation of the 

 trabeculse into the lateral cranial wall. As pointed out {antea, 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 fenestrse 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 fenestras 

 are in no way due to absorption, and that the trabeculse 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 conviction (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 



