DEVELOPMENT OF THE EAR IN THE COMMON EROG. 179 



in situ by the scattered cells in the vicinity aggregating into the form 

 of a bar. The position of the columella at this period is diagram- 

 matically represented in Fig. 14. It will be seen that it is not at 

 present in connection with the tympanic cavity or annular cartilage, 

 which are indeed far away, nor does it reach the outer surface of 

 the head by some distance. It is also directed much more directly 

 forwards than it is in the adult. 



Until metamorphosis takes place, this state of things remains 

 practically unaltered except by slight growth on the part of the 

 organs involved; but as the frog leaves the water, the columella 

 rotates with its base as pivot, so that it comes to point more outwards. 

 At the same time the mandibular and hyoid arches move back, as do 

 the tympanic cavity and annular cartilage. This movement brings 

 the end of the columella into close relation with the ventral surface 

 of the quadrate cartilage near its point of articulation with the capsule, 

 and an attachment takes place between the two by means of a thin 

 connecting bar, the suprastapedial. During this period the columella 

 grows rapidly m length and increases in thickness throughout its 

 whole extent. This causes the distal end to extend close over the 

 tympanic cavity and to come in contact with the annular cartilage 

 very shortly after the frog has completely lost its tail. At the same 

 time chondrification, which has hitherto only appeared at the 

 proximal end, spreads along the whole length (see Figs. 15 and 16). 

 The outer end of the columella now expands against the tympanic 

 membrane, and the cavity below, by growing upwards, causes the 

 cartilage to appear to project through the tympanic cavity on to the 

 membrane. Thus we have the permanent conditions established. 



It must be noted that the connecting bar between the meso- 

 stapedial and the quadrate cartilage chondrifies and persists, being 

 present even in the adult. 



The most interesting point connected with this section applies to 

 the homology of the columella. Firstly, it is not part of the hyoid 

 arch developmentally ; nor is it part of the mandibular ; for at its 

 first appearance it is connected with neither. The connection that 

 does take place occurs secondarily, and is not a mere temporary affair, 

 as is shown by its size and permanence. It is not to bo considered 

 as an ernbryological, but as an adult connection. The equivalence of 

 the columella with the hyomandibular bar, so strenuously maintained 



