

423 



prolongation. And I speak the more confidently on this point, be- 

 cause the delicate process of the notochord or cavity in the cartilage, 

 to which I have referred, contains opaque unchanged vitelline gra- 

 nules, and is therefore particularly conspicuous. The basal cartilage 

 is still divided by the notochord into two lateral moieties, which are 

 only united by a short band of cartilage in front of the end of the 

 notochord. It sends off from its outer side a cartilaginous process, 

 which envelopes the auditory capsule externally, but leaves on its 

 inner side a wide aperture for the entrance of the auditory nerve. 

 The oval auditory capsules thus formed have their long axes directed 

 outwards and forwards. 



The trabeculse are still better developed than before, but instead 

 of remaining distinct anteriorly, they have become fused together into 

 a single trapezoidal cartilage, which may be termed the ethmo-pre- 

 sphenoidal plate. This plate, as it were, divides anteriorly into two 

 flat, elongated and somewhat divergent processes, which are concave 

 downwards and end in truncate extremities. Fibrous tissue connects 

 the ends of these ethmovomerine processes with a crescentic carti- 

 laginous plate which supports the horny upper jaw of the tadpole. 



The posterior cms of the palatosuspensorial, or suborbitar, arch is 

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

 the auditory capsule ; but fort he 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 pos- 

 terior crus of the palatosuspensorial arch ; and that the optic tra- 

 verses 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. 



