250 rROCEEDixGS OF THE ACADEMY OF [March, 



takiDg place simultaneously iu the skeletal structures, it is neces- 

 sary, in order to follow the former, to obtain a right conception of 

 the latter. Hence in the present stage I will first treat of the 

 essential skeletal parts. In the first place, the axis of the quad- 

 rate extends in a more dorso-ventral direction than formerly, so 

 that noAv the mandibular articulation lies below the anterior edge 

 of the eye, instead of being entirely in front of it as before. This 

 position implies that the lower part of the quadrate has moved or 

 rotated backward through a slight angle. The processus quadrato- 

 crauialis anterior (pterygo-palatine) is now considerably elongated 

 in an antero-posterior direction, a change obviously associated with 

 the backward rotation of the quadrate. The processus muscularis 

 (orbital) begins to show signs of degeneration, especially along its 

 dorso-external edge. The hyoid still articulates to the ventral sur- 

 face of the quadrate. Posteriorly the processus ascendens has 

 degenerated and consequently the quadrate has lost its connection 

 with the wall of the brain-case, but instead it now joins by means 

 of its processus oticus the wall of the auditory capsule anterior to 

 the fenestra ovalis. In the membrane closing the latter the stapes 

 now appears as a large, oval mass of fully differentiated cartilage. 

 The columella auris is a delicate rod of primitive cartilage, closely 

 applied to the wall of the capsule. Anteriorly it terminates with- 

 out forming any connection whatever with either the quadrate or 

 any portion of the Eustachian tube and posteriorly it unites with 

 the stapes. The columella is most distinct and its tissue most 

 compact in its posterior portion, so that it cannot be regarded as 

 a derivative of the quadrate. 



As a consequence of the posterior rotation of the distal portion 

 of the quadrate, the distal extremity of the Eustachian cord is 

 now situated somewhat posterior to its former position, but its 

 relation to the immediately surrounding structures is the same as 

 before, since these likewise have been aff^ected by the quadrate's 

 change of axis. The annular cartilage, now a dense cellular mass, 

 is situated under the anterior margin of the eye and above the 

 mandibular articulation. To its ventral sui face the more anterior 

 fibres of the depressor ossis hyoidei have acquired attachment. 

 Imbedded in the cartilage is the distal end of the Eustachian cord, 

 the future tympanic cavity. The general appearance of the cord 

 is similar to that in the preceding stage. The cord still shows 



