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1874.] PROF. T. H. HUXLEY ON MENOBRANCHUS. 199 



In all the other genera, so far as I am aware, these structures are ossi- 

 fied to a greater or less extent in front of the exit of the optic nerves. 

 In Proteus, the ossification is internasal only ; in Siredon and 

 Menopoma, it is interorhital only ; in Siren, there are two extensive 

 interorbital ossifications, which send median prolongations into the 

 internasal septum, and thus afford a transition to the fully developed 

 sphenethmoid (or " os en ceinture " of Cuvier) of the Frogs. 



I see no reason for doubting the homology of the paired inter- 

 orbital ossifications of Siredon with the orbito-sphenoids of the higher 

 Vertebrata ; in which case, that portion of the basis of the skull 

 which arises out of the coalescence of these parts of the trabeculae, 

 and is ossified into one mass with them, in the Frogs, must represent 

 the presphenoid ; and if this be so, the floor and side-walls of the 

 skull, between the interorbital ossification and the pro-otic bones, 

 must answer to the basisphenoid and the alisphenoid ; while that 

 which lies in front of the interorbital ossification must correspond 

 with the median and lateral ethmoids of the higher Vertebrates. 



In the Amphibia all these parts are formed by the gradual exten- 

 sion and subsequent metamorphosis of the trabeculae. All the steps 

 of gradual enlargement, apparent outgrowth, and metamorphosis of 

 these primitively rod-like cartilages can be followed ; and no part of 

 the chondrocranium in these regions is formed independently of them. 

 This is all I intend to convey by the expression that the sphenoidal 

 and ethmoidal regions of the skull are products of the growth and 

 metamorphosis of the trabeculae. If the questions be raised, Have 

 the trabeculae, when once formed, a quasi-independence ? and do they 

 grow into the adjacent tissues, as a tree pushes its roots into the soil? 

 Or does not their extension and apparent growth arise rather from a 

 chondrification of the preexisting tissue in the immediate neighbour- 

 hood of the trabecular cartilage 1 it seems to me that no definite 

 answer can be given to them. 



In the larval Triton and Siredon, at the stage of development 

 described above, for example, there is no complete cartilage either 

 at the sides of the notochord, behind the trabeculae, or in the eth- 

 moidal region, in front of the trabeculae. And it would seem that 

 the cartilage which eventually exists in both these regions, arises 

 in the same way — namely, by gradual chondrification of the tissue, 

 beginning in that part which is in contact with the trabecula, and 

 extending backwards, or forwards, as the case may be. And it may 

 be said that if the apparent growth of the trabecula into the para- 

 chordal region is not to be described as a backward growth of the 

 trabecula, so neither is the alisphenoidal, or orbito-sphenoidal, carti- 

 lage in the side-wall of the skull to be described as an upward 

 growth of the trabecula ; and this view would receive support from 

 any cases in which the orbito-sphenoids, or alisphenoids, take their 

 origin by independent development in the side-walls of the skull. 



The same difficulty arises when we attempt to determine the 

 nature of the cartilaginous walls of the nasal chambers. To all 

 appearance these, in all Amphibia which possess them, grow out of 

 the coalesced trabeculae. But if it be said that they are independent 



