344 EMBRYOLOGY OF THE LOWEK VERTEBRATES ch. 



behind the cleft to its lower limit and then curves forwards beneath 

 the cleft towards the region of the lower jaw and floor of the buccal 

 cavity. 



The external opening of the spiracular cleft gradually closes, 

 from below upwards, as is usual with this cleft, a stage being passed 

 through in which only the dorsal end of the cleft is open— precisely 

 as in the adult of an ordinary Elasmobranch. As the lower limit of 

 the opening gradually shifts dorsalwards the chorda tympani remains 

 in close relation with it so that the portion of the nerve on the 

 ventral side of the opening assumes a more and more dorsal position. 

 Eventually even the dorsal vestige of the cleft closes so that the 

 spiracle has now reverted to the condition of a pouch. Owing to 

 the shifting in position of the chorda tympani as it followed the 

 retreating lower edge of the spiracular opening this nerve now 

 passes forwards dorsal to the main portion of the pouch, instead of 

 entirely ventral to it as it did originally. 



The dilatation of the outer end of the pouch to form the tympanic 

 cavity is brought about mainly by active growth of the lower 

 portion of its posterior wall. This bulges outwards and spreads 

 forwards and dorsalwards beneath the epidermis, from which how- 

 ever it remains for a time separated by a considerable thickness of 

 mesenchyme. Later on this thins out relatively so that the three 

 layers bounding the tympanic cavity on its outer side — endoderm, 

 mesenchyme, ectoderm — form a thin membrane — the tympanic 

 membrane. As the tympanic dilatation goes on expanding in a 

 dorsal and anterior direction the chorda tympani becomes displaced 

 in front of it still further from its original position. 



In the mesenchyme of the hyoid arch there takes place a gradual 

 condensation to form the rudiment of the skeletal a.rch. The lower 

 and main portion of this condensation becomes the cartilage of the 

 definitive main cornu of the hyoid. Its*dorsal portion also becomes 

 converted into cartilage, taking the form of a stout rod the inner 

 (" stapedial ") end of which fits into the fenestra ovalis — a vacuity 

 in the wall of the auditory capsule — while its outer portion 

 (" extra-columella," Gadow) extends outwards towards the skin, 

 embedded in the mesenchyme of the posterior wall of the spiracular 

 pouch or tympanic cavity. Finally the lining of this cavity grows 

 actively dorsally and ventrally to the columella so that it bulges 

 backwards both above and below the columella. The pockets of 

 tympanic lining so formed meet round the columella and fuse 

 together so that the columella, instead of being embedded in the 

 hind wall of the cavity, now passes right through it, enclosed in a 

 delicate sheath of mesenchyme covered with endoderm. The exten- 

 sion of the tympanic cavity backwards past the columella causes an 

 extension of the tympanic membrane in the same direction so that 

 the point at which the tip of the extra-columella reaches the skin, 

 instead of being situated behind the tympanic membrane as it was 

 originally, comes to be about the centre of that membrane. 



