v AUDITORY SKELETON 345 



The general mode of development of the columella and the 

 cavities associated with it as seen in Lacerta appears to he typical of 

 the Sauropsida in general. It is now necessary to refer to a few 

 additional details. 



The inner end of the columella (stapes) fits into the fenestra 

 ovalis. It is for a time, during prechondral or cartilaginous stages 

 or both, continuous with the wall of the auditory capsule and is 

 probably to be interpreted as a portion of this wall which has 

 become separate and movable. 



Chondrification of the columella commences in the Lacertilia 

 from three centres according to Versluys and it is to be noted that 

 the separation between extra-columella and stapes arises secondarily 

 within the region of cartilage which develops from the innermost 

 centre. 



In Birds an interesting variation has been discovered (Goodrich, 

 1915) in the relations of the chorda tympani. In the Duck these 

 are normal, agreeing with what has been described for Lacerta. In 

 the ordinary Fowl and the Turkey however the stage in which the 

 chorda tympani is posterior to the hyomandibular cleft is omitted 

 from development. Even in early stages it is found to pass in front 

 of the pouch or cleft. 



This is one of those cases which emphasizes the need of caution 

 in regarding the course of a nerve as a necessarily deciding factor in 

 discussions as to the morphological nature of particular organs. 

 Position in regard to a particular nerve-trunk often affords us most 

 valuable evidence regarding the primitive position of an organ. 

 Here, however, we have it impressed upon us that we must never 

 rely absolutely upon such a piece of evidence taken by itself. Were 

 we to do so in this case we should be led into the absurdity of con- 

 cluding that the tympanic cavity of the Turkey is not homologous 

 with that of the Duck. 



As a matter of fact nerve-trunks do not always form impassable 

 barriers to the evolutionary change in position of organs. A skeletal 

 structure may spread round a nerve-trunk {e.g. neural arches of 

 Dog-fish) and becoming absorbed behind it may come to be transposed 

 entirely past the nerve. In the case of the chorda tympani and the 

 tympanic cavity it is clear that the nerve lay primitively behind and 

 below the cavity and we may probably take it that, in accordance 

 with the general principle that nerve-trunks tend to shorten and so 

 economize material, in the course of evolution it became shifted 

 dorsalwards through the mesenchymatous middle layer of the outer 

 wall of the tympanic cavity before it became thin and membranous, 

 so as eventually to lie completely dorsal and anterior to the tympanic 

 membrane. 



Incidentally the variation from normal development occurring in 

 the Turkey and Fowl is one of those cases apparently impossible to 

 explain on the outgrowth theory of nerve-development, but readily 

 understandable on the view of nerve - development supported in 



