DEVELOPMENT OE THE SKULL OF THE COMMON FROG. 
183 
it once did forwards, so that the angle formed by the suspensorium with the basicranial 
axis is very obtuse (see Plate IX. fig. 3, q.). The ectosteal plate of the quadrate is 
overlapped in its broad grafting portion by the spatulate lower end of the squamosal ; in 
front of the cartilage the quadrate plate grows into a long quadrato-jugal process which 
largely overlaps the long jugal process of the maxillary. 
The posterior lobe of the quadrate condyle is, in reality, the “ symplectic lobe ” (sy.) ; 
but, as in the palato-pterygoid, the cleft has filled-in again*, which for a time separated 
it from the mass of the quadrate : this return to a lower type seems to be peculiar, but 
must be caused by some general law. 
The rest of the mandible has changed very little from its condition in the last stage* 
The “ intermaxillary rudiment ” or “ mento-meckelian ” (Plate IX. fig. 3, m.mk .) is 
permanently small, and the dentary runs back two-fifths of the length of the ramus ; 
the “ articulare ” ensheaths the inner side completely, but a wide space of the perma- 
nently unossified core is bare on the outside. The articular condyle ( ctr.c .) is a smooth 
egg-like mass, with its long axis longitudinal ; it rolls very freely beneath the smoothly 
scooped base of the quadrate. 
The pier of the next arch has been differentiated into most of the structures of the 
middle ear, as well as the hyoid crus ; it has been subjected to a large amount of special 
metamorphosis, the hyoid arch lending, as it were, much of its substance to the organ 
of hearing. 
When the parts which have grown by modification of the suprahyomandibular seg- 
ment are examined in situ (Plate IX.), it is seen that they reach from the stapedial 
plate (st.) to near the anterior edge of the membrana tympani (Plate IX. fig. 3, m.t., 
s.st.). Seen from behind (fig. 4) the whole series is arcuate ; for the “ medio-stapedial ” 
ascends a little, and the extrastapedial descends in the same degree ; but the principal 
direction of the former is outwards , and of the latter forwards, whilst the feeble “ supra- 
stapedial” ray passes upwards, backwards, and inwards, and is loosely attached to the 
“ tegmen tympani ” by a delicate fibrous ligament. In the enlarged side view (Plate 
VIII. fig. 9) the whole series of “ middle-ear ” elements is drawn out, longitudinally, for 
display, the “ tegmen ” being pared away, the osseous medio-stapedial ( m.st .) being bent 
towards the skull, and the cartilaginous parts freed from their attachments. The “ supra- 
stapedial ” (s.st.) is a delicate rounded rod continuous with the “ extrastapedial ” below, 
and forming an acute angle with the medio-stapedial ” (m.st.) ; it is the “ anterior hyo- 
mandibular fork ” modified (see Plate VI. fig. 8). The “ extrastapedial ” (e.st.) is an 
elegant spatula ; it is thin, convex on the outer and concave on the inner side, and on its 
convex side it receives the fibrous mesh of the “membrana tympani” (Plate IX. fig. 3). 
At first sight it would seem strange that this free cartilaginous spatula should answ r er to 
the outstanding condyle on which the “principal opercular ” is articulated in the Osseous 
Pish ; but a reference to the early condition of these parts in both Fish and Frog will 
make things clear. If my fourth stage (Plate V. figs. 1-4, and Plate VI. fig. 8) be 
* Here we have what may he called retrograde metamorphosis. 
