254 PROCEEDINGS OF THE ACADEMY OF [jMaVcll, 



like the advilt structui-e. Figs, 30 and 31 illustrate the condition 

 of the tube at this time. In fig. 30 we have a ti-ansverse section 

 through one side of the head immediately back of the eye. To the 

 outer side is shown the quadrate cartilage, which in the present 

 stage stands almost vertical and hence is shown in the figure cut 

 throughout the greater part of its length. External to the upper 

 portion of the cartilage is the tympanic portion of the Eustachian 

 tube, showing a slight lumen (Tijm.). I have another specimen of 

 approximately the same age in wdiich the lumen is much larger, 

 forming a considerable cavity. Underlying this portion is the 

 tympanic or annular cartilage, which in its ventral portion, at 

 least, is completely chondrified {An.). The fully formed car- 

 tilage does not, however, form a complete ring. Internally the 

 tube approaches the outer surface of the quadrate, as was the case 

 in the earlier stages. Applied to the dorso-external wall of the 

 tympanic cavity is the distal extremity of the columella auris, at 

 present a very compact cellular mass, not yet differentiated into 

 true cartilage {CI.). The apparent inclusion of the columella 

 within the tympanic cavity is produced by the subsequent growth 

 of the latter around this portion of the cartilage. Attached to 

 the ventral surface of the annular cartilage are fibres of the 

 depressor ossis 'hyoidei {m.d.h.). The attachment of the muscle 

 to the cartilage was acquired soon after the earliest Anlage of the 

 latter had appeared. At its ventral end the muscle has lost its 

 attachment to the hyoid cartilage and has acquired a new insertion 

 into the angle of the mandible, so that like the depressor man- 

 dibulse it serves to depress the latter (compare also fig. 31). The 

 bulk of the muscle lies posterior to the Eustachian tube. Internal 

 to the muscle and between it and the quadrate are two blood- 

 vessels, which correspond to the original mandibular aorta {m.a.). 

 This vessel, as we have seen, was an important, one during the 

 tadpole period, but during the metamorphosis it underwent some 

 profound changes. Its middle portion largely degenerated, so 

 that the vessel became divided into a proximal and a distal half. 

 The vessel undergoes other changes, but these I have not been able 

 to follow satisfactorily with the material at hand. 



Fig. 31 shows a section thi-ough the tympano-Eustachian tube 

 near its posterior boundary. The quadrate {q. ) is here seen in 

 two separate portions, a dorsal and a ventral. This condition can 



