DEVELOPMENT OF THE SKULL OF THE COMMON FROG. 
163 
Within the nostril there is a small point of bone, the septo-maxillary ( s.mx .) ; this will 
be better seen in the next stage. 
The vomers have not yet appeared, but delicate spicnlse of bone are found in the 
fibrous stroma, lining the inner face of the suspensorium and of the pterygoid bars ; these 
will be best described in the next stage ; and when they are better developed they be- 
come the pterygoids, metapterygoids, and infrahyomandibulars. 
On the outer face of the suspensorium a much better-developed plate of bone has arisen 
(t); it is sigmoid in shape, broadish above, and pointed below; this resembles the 
“ praeoperculum,” but it does not free itself from the common root of the mandibular 
and hyoid arches to grow backwards ; it acquires a transverse “ supratemporal” portion, 
a separate bone in Triton , and thus has a right to the title “ squamosal ” or “ tem- 
poral.” 
On Mekcel’s cartilage, also, membrane-bones have appeared, these are the dentary (d.) 
and the articular (ar.) ; at this stage there is no difference in the character of these bones ; 
they are thin fenestrate plates lying in contact with the cartilage, or at least having 
only a slight layer of fibrous stroma between them and the rod. 
The development of the cranium has been very rapid. The trabecular “ horns” have 
united in front to form the lower nasal alae ; the lateral cranial walls, forming the 
ethmoidal, orbito-sphenoidal, and alisphenoidal regions, are now well grown (Plate VII. 
fig. 3, eth., o.s., al.s.). 
The cranial roof, also, under the parietals, and beneath them and the frontals, is now 
roofed-in with an extension forwards of the pterotic cartilage (pt.o.) : here we see the 
posterior half of the great upper fontanelle covered in. 
The suborbital space or fenestra ( s.of ) is greatly changed in form by a reversal of the 
relative length of the crura which bound it (compare Plate V. figs. 1-4 with Plate VII. 
fig. 3) ; it is also much longer from end to end. Anteriorly, the palatal region (pci.) 
is nearly segmented off by a transverse cleft from the pterygoid bar ; and in front of this 
joint it has developed a knee-like enlargement. The pterygoid bar (pg.) is a very uni- 
form flat band until it reaches the suspensorium, which part is rapidly undergoing meta- 
morphosis. The metapterygoid root has now coalesced with the fore part of the periotic 
capsule. The “ orbitar process” ( or.p .) is now reduced to a gently convexo-concave ex- 
pansion of the fore part of the suspensorium ; below, it belongs to common ground be- 
tween the metapterygoid and quadrate regions, and is almost entirely behind the temporal 
muscle. 
These parts are well seen from the inner side (Plate VII. fig. 4). The lower metaptery- 
goid region as it passes into the quadrate (• m.pg ., gw.) is of considerable thickness; but 
the starved orbitar process grows out of it in front like a thin shell, its concave face in- 
wards. But behind the middle thick part there is another shell-like plate of cartilage 
( i.hm .) ; this also is thin, but it has its convex surface on the inner side, and is deeply 
scooped externally; this is the “ infrahyomandibular,” between which and the meta- 
pterygoid band there is still the remains of the upper part of the first cleft. 
z 2 
