■576 ON THE THEORY OF THE VERTEBRATE SKULL 



trabecula close to the basal plate, while at its distal end it sends a 

 prolongation upwards to unite with the posterior end of the ethmo- 

 vomerine cartilage. It then forms an arch, between which and the 

 basis cranii is an interspace corresponding with, and lodging, the 

 under surface of the large eyeball. The rudiments of the hyoid, 

 mandibular and maxillary apparatus in larvae at this stage are some- 

 what indistinct ; and indeed not only in this, but in other respects, 

 more instruction is to be derived from tadpoles which have advanced 

 further. 



In larvae, with completely internal branchiae and very short 

 tubercles in the place of hind limbs, the notochord suddenly narrows 

 between the auditory capsules to hardly more than half its preceding 

 dimensions, and then gradually tapers off, to what appears to be a 

 rounded end, a short distance from the anterior boundary of the 

 basal plate. On very careful examination, however, a delicate 

 process (which may by possibility be nothing but a cavity in the 

 ■cartilage) can be traced from it very nearly to the margin of the 

 basal plate. But there is no continuation whatsoever, either of the 

 notochord itself or of its sheath, into the subpituitary membrane, 

 which is now composed of delicate connective tissue, and from its 

 ' extreme thinness and transparency would exhibit the least trace of 

 such a prolongation. And I speak the more confidently on this 

 point, because the delicate process of the notochord or cavity in the 

 cartilage, to which I have referred, contains opaque unchanged 

 vitelline granules, and is therefore particularly conspicuous. The basal 

 cartilage is still divided by the notochord into two lateral moieties, 

 \\-hich 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 axis 

 directed outwards and forwards. 



The trabeculae 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- 

 presphenoidal 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 

 cartilaginous plate which supports the horny upper jaw of the 

 tadpole 



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



