190 
ME. W. K. PAEIvEE ON THE STBTJCTUBE AND 
delicate terete rod which passes upwards and backwards, and is attached to the “ tegmen 
tympani ” by a fibrous ligament; this is the “ suprastapedial ” (s.st.). The freed “ oper- 
cular lobe ” has become an elegant spatulate “ extrastapedial ” (e.st.) ; and it is tilted 
more upwards than when it was unsegmented in the fourth stage (fig. 14). The hinder 
head of the “ hyo-mandibular ” has bent itself downwards a little, has become relatively 
very long, and has become ossified into a little bony shaft. The “ condyle ” at its free 
end has become segmented off into an “ orbicular element,” the “ interstapedial ” ( it.st .) ; 
the bony bar is the “ medio-stapedial ” ( m.st .). 
Ninth Stage (fig. 19). — In three months more the facial bars are becoming more and 
more like those of the adult, the pterygo-palatine and Meckelian rods larger still ; and 
as a correlate of this, the suspensorium now forms an obtuse angle with the basicranial 
axis. The “ infrahyomandibular ” ( i.hm .) now forms an elegant hatchet-sliaped condyle, 
which glides over the antero-inferior face of the periotic cartilage directly inside the 
metapterygoid root. The elements superadded to the stapedial are lengthened, are 
more elegant in form, and more completely adapted to the proper stapedial plate. A 
long sinuous ligament still binds the stylo-hyal head to the inferior angle of the “ infra- 
hyomandibular but a new ligament ties it to the periotic capsule below the stapes. 
Tenth Stage (fig. 20). — We now come to the skull of the old Frog, where we see what 
is typical in the highest form of the Batrachian. The first or trabecular bar (1 tr.) still 
shows the tips of these “horns” on the front of the nasal box; but, for the rest, they 
are completely amalgamated with the nasal and cranial structures. Between them and 
the second bars (first postorals, gii.) there now intervenes the long, elegantly arcuate, 
fore-spurred pterygo-palatines (p a -pg.) > an( l yet this second bar, to which the middle 
third of the third bar has coalesced, has much the same curve in itself, and has a similar 
angular relation to the basicranial axis as in the first stage (compare figs. 11 & 20). Thus 
the mandibular pier, leaving out of consideration the free Meckelian rod, has gradually 
travelled so far forwards as to be nearly parallel with the basis cranii (fig. 14), and then 
has slowly grown backwards until it has more than regained its original position. The 
middle third of the third (second postoral, i.hm .) still retains its suspensorial function, 
although it has long lost its distinctness. Having been carried upwards during growth, 
it reaches just to the point where the apex of the arch was in the third stage (fig. 13, h.m.), 
when the auditory sac had grown so far forwards as to overshadow the bar. Having lost 
the upper third, which was segmented off to form the uniting chain, it now articulates 
by a gliding joint (Plate IX. figs. 2, 4, 7, i.hm.) with that same region of the ear-sac ; and 
the mandibular pier, having clung to the much-projecting cartilage of that sac, is now 
exactly outside this remnant of the third bar. The 4- cornered free hyoid segment, once 
so far from the ear-sac, and being indeed the lowest two-fifths of the third bar (fig. 13), 
is now a long band articulating with the ear-sac a little way behind the head of the middle 
part (“ infrahyomandibular ”), from the base of which it was first taken (fig. 13 & fig. 20). 
The highly metamorphosed upper end of the second postoral bar now forms four out of 
five of the elements of the “ middle-ear ” chain. The stapedial plug (st.) and the ter- 
