422 



That part of the middle of the hasis cranii which underlies the 

 pituitary body is not converted into cartilage, but remains mem- 

 branous, and may be called the " subpituitary membrane." The 

 delicacy of this membrane is so great that it is easily torn, when 

 the pituitary body seems, as Rathke originally supposed, to unite 

 with the palatine mucous membrane. But that this is not really 

 the case, is readily demonstrable in an embryo whose tissues have 

 been sufficiently hardened with alcohol or nitric acid. 



The cartilaginous basal plate gives off a prolongation on either 

 side of the subpituitary membrane. This, the " cranial trabecula" 

 (Schadelbalke of Rathke), passes forwards with a slight convexity 

 outwards, arid then turning inwards comes into contact with its 

 fellow (from which, however, it is at first distinct), and spreads out 

 into a broad, flat, elongated process, which I shall term the ethmo- 

 vomerine cartilage. 



Behind the eye and just in front of the auditory capsule (in the 

 posterior part of the first visceral arch, therefore), a cartilaginous 

 process lies, which is connected proximally with the root of the tra- 

 becula 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, mandi- 

 bular and maxillary apparatus in larvae at this stage are somewhat 

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

 struction is to be derived from tadpoles which have advanced further. 



In larvae, with completely internal branchiae and very short tu- 

 bercles 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 pro- 

 cess (which may by possibility be nothing but a cavity in the car- 

 tilage) can be traced from it very nearly to the margin of the basal 

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

 chord 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 



