ON THE THEORY OF THE VERTEBRATE SKULL 57/ 



not yet united with that portion of the cranial wall which encloses 

 the auditory capsule ; but for the rest the same description applies 

 to it which has already been given of the palatosuspensorial arch and 

 its appendages in more advanced tadpoles. In this state, the roof, 

 and all the lateral walls of the cranium, but that part into which the 

 auditory capsule enters, are membranous. 



If the skull of the larval frog just described, be laid open and the 

 exit of the nerves observed (fig. 9), it will be seen that the par 

 vagum makes its way out by a foramen situated immediately behind 

 the auditory capsule ; that the third division of the trigeminal leaves 

 the cranium in front of the auditory capsule, passing over the posterior 

 crus of the palatosuspensorial arch ; and that the optic traverses the 

 membranous walls of the skull between this and the olfactory nerve, 

 which perforates the anterolateral region to enter the olfactory 

 capsules. The latter are situated wide apart, on each side and in 

 front of, the broad ethmo-presphenoidal cartilage and the anterior 

 crus of the palatosuspensorial arch, and are even a little overlapped 

 by the edges of the ethmovomerine processes. 



In the further course of development, the trabeculae approximate 

 and elongate, so as to obliterate the subpituitary membrane, and 

 form with the enlarged basal cartilage, the ethmoid cartilage and the 

 ethmovomerine cartilages, the continuous cartilaginous craniofacial 

 axis. A histological metamorphosis into cartilage is undergone by 

 the roof of the occipital region of the skull, but in front of this it 

 remains membranous ; so that in the adult frog (in which this 

 cartilaginous framework persists), the skull, when deprived of its bony 

 matter, presents an anterior fontanelle. The ethmovomerine cartilages 

 diverge still more, and form the broad mass whose lateral cavities 

 shelter the olfactory sacs in the adult frog. If, bearing in mind the 

 changes which are undergone by the palatosuspensorial apparatus, 

 and which have already been described, we now compare the stages 

 of development of the frog's skull with the persistent conditions of 

 the skull in the Amphioxiis, the Lamprey, and the Shark, we shall 

 discover the model and type of the latter in the former. The skull 

 of the Amphioxus presents a modification of that plan which is 

 exhibited by the frog's skull, when its walls are still membranous 

 and the notochord is not as yet embedded in cartilage. The skull 

 of the lamprey is readily reducible to the same plan of structure as 

 that which is exhibited by the tadpole, while its gills are still external 

 and its blood colourless. And finally, the skull of the shark is at 

 once intelligible when we have studied the cranium in further advanced 

 larvae, or its cartilaginous basis in the adult frog. 



p p 



