DEVELOPMENT OF THE SKELETON OE THE TL'ATAEA, 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 trabeculse in Sphenodon is that of Huxley, originally put forward in 

 his Hunterian Lectures in 18G4, that they represent a pair of prse-oral visceral arches. 

 Huxley reiterated this conclusion in 1874, in his suggestive note on Amphioxus i, and 

 von Kiipffer gave it consideration and support in 1893 2, in his discovery of a pair of 

 pree-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, maxillse, postorbitals, squamosals, angularia, supra-angularia, and dentaries 

 are all present (PI. III. fig. 5) and remote from each other. By the time that R is 

 reached, all the membrane-bones present in the adult are represented {cf. PI. IH. 

 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 prsefrontal, 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 skuU is viewed either from above (fig. 11) or the side (fig. 10). And one is 

 led to speculate hoAV far this may not be indicative of a first protection of the ej^e, 

 when it is remembered that the palseontological record proves the circumorbital 



' Huxley, T. H. : Proc. Koyal Soo. vol. xsiii. 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 skuU. He was bringing to bear upon it the discovery in the Elasmobranohs of a fourth 

 branch of the trigeminal nerve, to have been termed the j^alato-nasal or hyporhinal. This term was intended 

 to express the fact that, in relation to the trabeoulaj, this branch of the fifth cranial nerve, together with the 

 ophthalmic (for which he had already introduced the correlative term orhiio-nusal, 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 pra3-oral clelt. 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. vergl. Entwick. Kopfes d. Kranioten, Hit. i. Acipenser : Muncheii, 1893, p. 89. 



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