DEVELOPMENT OF THE SKELETOIS^ OF THE TUATARA. 39 



further fenestrse arise in the same vertical plane in front of those already described. 

 Of these the upper or lateral cranial fenestra {f.n., PL III. figs. 4, 6, 7) is wholly 

 membranous, transmitting nothing, the lower (/. ii.) giving passage to the optic nerve. 

 It will further be noted that with the establishment of this union between the ethmo- 

 and otosphenoidal plates the parts most nearly related to the latter have become 

 expanded and carried up by growth of the basi-ethmoidal plate, together with which 

 they constitute a considerable cartilaginous interorbital septum. 



The fourth otosphenoidal process, originally directed downwards (os.*, fig. 1), now 

 carried up, looks straight backAvards {cf. figs. 5 »& 8), but it has remained free. 



On appreciation of these important processes, which have effected so marked a 

 change in the configuration of the parts, it is evident that growth must be most rapid 

 in the otic border of the otosphenoidal plate. 



The chrondrocranium at this stage, viewed from beneath, reveals changes which, 

 though less dominant than those described above, are in themselves remarkable. 

 Massive indeed have the basis cranii and auditory capsules become, under the accelerated 

 growth associated with the straightening of the basi-cranial axis and the marked 

 lateral expansion of the post-pituitary floor {cf. figs. 6 & 7). The occipital region 

 has now become conspicuous, by the upgrowth on either side [e.o., PI. III. figs. 6 & 7) 

 of a considerable cartilaginous lobe lying free of the auditory capsule, between which 

 and it there lies the huge interspace for the 9th, 10th, and 11th cranial nerves (figs. 6 

 & 7,/'. ix. to xi.). 



Anteriorly to the trabeculse the basal ethmoidal plate {cf. fig. 6), vertically extended 

 as has been seen, has undergone a forward growth — as is proved by the fact that its 

 length is now as great as that of the trabeculae themselves, and it has also undergone a 

 downward rotation, very marked as viewed from the side (PL III. fig. 4). 



The pituitary foramen at this stage {py.', PL III. figs. 6 & 7), compared -with that 

 of Stage P, as represented in the accompanying figs. {cf. fig. 2), would seem to have 

 undergone considerable reduction with advancing growth. When, however, it is 

 remembered that the figure of Stage P (fig. 2) is magnified 13 times, that of Q 

 (figs. 6 & 7) but 8, it will be evident that under the growth-changes going on the 

 trabecular tracts which more directly bound the foramen have been well-nigh stationary, 

 and that such reduction of the foramen as has taken place would appear to be rather 

 due to the mode of union between the trabeculse and parachordal cartilages than to 

 actual cartilaginous invasion. 



Finally, as to the sense-capsules— now both chondrified. With the olfactory capsule 

 complication has set in. Its capsular portion {nc.) has entered into a union with the 

 anterior border of the ethmosphenoidal plate, giving off dorsally a short tiiq^ranasal 

 process {ncJ, fig. 4) connected with the plate by an ethmonasal bar {e.b.) lying within 

 the outer border of the olfactory nerve foramen. At its anterior extremity it gives oft' 

 a prcenasal process {nc!', fig. 4), which embraces the anterior wall of the nasal sac. 



