DEVELOPMENT OF THE SKELETON OF THE TUATARA. 51 
Far reaching as is his discovery, and interesting as are its bearings on our own 
work, we venture to think that the most rational interpretation to be put upon the 
behaviour of the trabecule in Sphenodon is that of Huxley, originally put forward in 
his Hunterian Lectures in 1864, that they represent a pair of pree-oral visceral arches. 
Huxley reiterated this conclusion in 1874, in his suggestive note on Amphioxus 1, and 
von Kiipffer gave it consideration and support in 1893, in his discovery of a pair of 
pre-oral visceral diverticula in the embryo Sturgeon. Further consideration of this 
very important topic is beyond the possibility of this memoir, but we hail with delight 
the discovery, as our work was nearing its termination, by von Davidoff, of the 
existence of this very pair of diverticula in the Lizards Platydactylus and Lacerta. 
Cranio-facial Membrane-Bones. 
The anatomy of these has been so fully dealt with by various authors, and most 
recently by Osawa (98*. p. 497), that we have little to add to their descriptions. 
Developmentally, however, some interesting considerations arise. At Stage Q, the 
earliest at which we have been able to observe membrane-bones, the vomers, palatines, 
pterygoids, maxille, postorbitals, squamosals, angularia, supra-angularia, and dentaries 
are all present (PI. ITI. fig. 5) and remote from each other. By the time that R is 
reached, all the membrane-bones present in the adult are represented (ef. Pl. ITI. 
figs. 10-12). Looking at the skull from the side, this does not at first sight appear 
so, but when examined fully it is found that while the frontals are approximated in 
the middle line, the parietals are widely divaricated and small and of a remarkable 
angulated type. Examination of the skull at this stage shows that the frontal and 
postfrontal, the prefrontal, maxilla, jugal, and postorbital are all in close relation- 
ship, first active development having apparently involved those elements which are 
circumorbital—a fact which immediately impresses itself on the mind when, at this 
stage, the skull is viewed either from above (fig. 11) or the side (fig. 10). And one is 
led to speculate how far this may not be indicative of a first protection of the eye, 
when it is remembered that the paleontological record proves the circumorbital 
‘ Huxley, T. H.: Proc. Royal Soc. vol. xxiii. 1874, p. 131. 
There is evidence among Huxley’s unpublished notes that in the early 80’s he was returning to this conception 
and to work upon the skull. He was bringing to bear upon it the discovery in the Elasmobranchs of a fourth 
branch of the trigeminal nerve, to have been termed the palato-nasal or hyporhinal. This term was intended 
to express the fact that, in relation to the trabecule, this branch of the fifth cranial nerve, together with the 
ophthalmic (for which he had already introduced the correlative term orbito-nasal, in his article ‘* Amphibia,” 
Encyclop. Brit., Vol. i. Edit. 9, p. 767), repeats the condition of the other cranial nerves in relation to their 
visceral arches and clefts, and thereby presupposes the existence of a pree-oral cleft. And, further, he was 
building up an argument on this basis to show that in these facts there lies the explanation of the mode 
and point of termination anteriorly of the notochord.—G. B. H. 
* Kiipffer, C. von: Stud. d. yergl. Entwick. Kopfes d. Kranioten, Hit. i, Actpenser : Miinchen, 1893, p. 89. 
H 2 
