308 



ME. W. K. PAEKER ON THE STEUCTUEE AND 



portio dura (7"), and mesiad of the jugular vein the glossopharyngeal nerve (8") is cut 

 through ; outside the nerve, and below the stylohyal (st.h.), a part of the exoccipital 

 (e.o.) appears. A similar section to the other side is also shown as a front view (fig. 8) ; 

 and here the relation of the portio dura (7") to the hyoid arch is well seen. This figure 

 does not show the incus, which is removed to display a new segment {i.liy.) that has 

 arisen, the counterpart of the Sauropsidian " infrastapedial " (H.) and of the so-called 

 " styloliyal" of the Fish (Cuv.) ; here it will be called the "interhyal " (P.), as it really 

 wants a proper name. In the second stage (Plate XXX. fig. 6) I have displayed the 

 relation of the segmenting second arch to the very Batrachian stapes (see "Frog's Skull," 

 Plate VII. figs. 12-16, st.) ; and on the same Plate (fig. 8) I have put for comparison 

 the state of things in the third stage; both are front views, and the apex of the stapes 

 is towards the eye. The bigeminal papulae on the younger stapes are now connected by 

 a bridge of cartilage, and the lateral dimples are the foot-hole of the little stirrup. 



The round head of the short crus of the incus is seen articulating with the tegmen 

 tympani (Plate XXX. fig. 8, t.ty., i.) ; and helow and behind it is the clavate head of the 

 stylohyal, which is ready to coalesce with the periotic cartilage at the junction of the 

 epiotic and opisthotic regions (see Plate XXXVI. fig. 2, i.,stJi., oj>., ejo.). The descent 

 of the dislocated hinder half of the shoulder of this second arch is not so great as in the 

 Osseous Fish ("Salmon's Skull," Plate vi. fig. 2), but it is considerable; and this is a 

 true third stage, as may be seen by comparing fig. 9, hi/,, in Plate XXIX. with figs. 

 6 & 8, Plate XXX. The binding, intervening band of new indifi'erent tissue which 

 has grown in the gap of these divided parts has acquired a hardening nucleus of new 

 cartilage, exactly as we see it in Ganoid and Osseous Fishes, e. g. Accipenser, Anguilla, 

 Sahno. The portio dura (7") is seen passing down its aquedvict behind these segments, 

 and the upturned, inbent, long crus or neck of the rib-like bar ends in an elegant sucker- 

 shaped disk, its capitulum or apex*. 



On the same Plate the postoral arches of the third stage are shown in a side view 

 (Plate XXX. fig. 9); and a comparison of the undivided mandibular bar with the displaced 

 fragments of the hyoid will make things plain to the mind. The apices of the two 

 bars come very near together ; but whilst the first hooks backwards, downwards, and 

 inwards, it does not graft itself upon the auditory sac ; nor does its shoulder send back- 

 wards a secondary pedicle so distinct as the " short crus of the incus." Moreover the shaft 

 of the bar on the first arch keeps on its way normally, as in the early embryo ; but in the 

 second arch this part has been segmented off", and displaced backwards and downwards, 

 catching in its descent at the neck and head of the arch, but travelling still further in 

 more advanced stages, until it rests and combines with the postero-inferior angle of the 

 auditory mass. In the new web which grows between the two segments comes the 

 secondary " interhyal " segment ; this, however, loses all its first relations, and finally 



* In Ijotli these figures, put together for comparison, tlie parts of the second arch are coloured, and those of 

 the auditory capsule are plain, for the sake of distinction, tliat the e>jc may learn to separate the "stapes" 

 from the segments of tlic hyoid arcli. 



