DEVELOPMENT OF THE SKULL OF THE COMMON FROG. 
173 
retains its separateness, and is now seen as an ovoidal shell lining the lower two-thirds 
within ofth e free “ infrahyomandibular lobe” (tig. 8", i.li.m.). 
The “ mento-meckelian” bone (figs. 7 & 8, m.m.Jc.) is nowin a very characteristic state, 
with its dentary continuation (cl.) along the outside of the Meckelian rod ; the “ articular 
ectostosis” (ar. ) is now seen to be of great length, reaching on the inside nearly to the 
“ mento-meckelian shaft.” 
The prootics and exoccipitals (pro., e.o .) are much extended into the surrounding carti- 
lage, and form complete rings to the fifth and eighth nerves. 
A new osseous centre has also made its appearance, and one of extreme interest, as it 
is especially a JBatracldan bone; this is the “ethmoid,” or “os en ceinture.” It com- 
mences where the ethmoidal cartilage forms a narrow transverse ledge to the front of the 
boat-like skull, the anterior boundary of the great fontanelle (fig. 7, eth.). This bone 
appears to have both its ectosteal and its endosteal portions formed quite synchronously; 
at present no trace of it can be seen from the sides or from below (fig. 8). 
After the fashion of the “ Placoidei,” the cartilaginous skull has begun to acquire bony 
matter ; but this direct calcification of the peripheral cells of the hyaline cartilage does 
not take the form of tesserae as in them, nor of proper morphological regions as in the 
higher types ; it does, however, tend to crowd itself where, in another type, a proper bone- 
territory would be established. 
This initial allotment is best seen in the epiotic region (<?».), but it can be traced on 
the superoccipital (s.o.), on the pterotic, on the basisplienoidal, and on the upper and lower 
nasal regions ; this “ superficial endostosis” can now be beautifully seen, the bony deposit 
being in very small grains — in semilunes half round a cell, and in separate and con- 
nected rings. 
Looking at the primordial skull as a whole, we see at this stage (compare figs. 7 & 8 
with figs. 1 & 2) that the cranial cavity is less oblong (it is now broader behind), and 
that the more extended periotic masses are relatively shorter antero-posteriorly. 
The “alinasal” folds (fig. 7, al.n.) have lost their articulation with the ethmoid; the 
“ prepalatal” bar is more extended and acute ; the whole subocular arch is now more ele- 
gantly arcuate, and extends much further backwards. But the suspensorium has changed 
most ; for it now turns backwards instead of forwards, forming an obtuse angle with the 
basicranial line. The extension backwards of the quadrate condyle is accompanied by 
much elongation of the mandible (figs. 7 & 8, m.k .), and the gape is now very similar to 
what is seen in the Crocodile. 
The upper part of the suspensorium is still very distinctly seen to be double (see 
Plate VIII. fig. 8", where the parts are somewhat drawn out for display, and the retrac- 
tion, so to speak, of the quadrate angle, has brought the two heads of the suspensorium 
into the same vertical line), the infrahyomandibular (i.hm.) now lying directly inside the 
metapterygoid band (m.pg.). But it is the metapterygoid which has changed its place, 
by what I have already described, namely by fusion of the upper part of its root with 
the fore face of the auditory mass, whilst the “infrahyomandibular” has attained such 
