DEVELOPMENT OF THE SKULL OP THE COMMON FROG. 
167 
round the greater part of the opening behind ; and then the cartilage ends in a broad 
process which abuts against the snout-cartilage (sn.c.). If we now consider the manner 
in which the roof grows down into the side wall , it will be observed that there is a 
large swelling pouch on each side, connected by a broad tract with the aliseptal plate 
( al.s .), but sharply distinct from the aliethmoid ( al.e .). Indeed, close to the ethmoid 
this pouch seems to be undergoing dehiscence, so as to separate the side-wall from 
the roof. These lateral pouches are the “ alinasal” cartilages ( al.n .) ; they terminate 
on the underside by curving in a falcate manner round the internal nostril (Plate VIII. 
fig. 2, i.n.). 
The falcate process nearly applies itself to the elegant semicircular space between the 
outer edge of the trabecula, and the anterior margin of the palatine bar (s.n.l., pa.) ; this 
is the normal position of the internal nostril (see Plate IV. fig. 10, and Plate V. fig. 4, ol.). 
A triangular fibrous valve is seen within the internal opening (Plate VIII. fig. 2, i.n.). The 
passage itself is very correctly circular. The alinasal pouches are very large (relatively) 
at this stage. 
Mesiad of the inner nostril there is a delicate tract of ossified fibrous tissue, spreading 
like a patch of small crystals; this is the “vomer,” ( v .) related by its outer edge to the 
inner nostril (i.n.). 
The peculiar position of the symmetrical vomers and the extreme distance of the inner 
nares, depends upon their relation to the “ trabecular horns.” If these coalesce to form 
merely the thickened base of the septum nasi, then the vomer is generally azygous, and 
the palatal openings of the nasal labyrinth are merely separated by the thickness of the 
septal base and the width of the azygous vomer. In the Frog, however, we have not 
merely the primary width of the diverging trabecular horns, but also their after-spreading, 
which gives still further lateral extension to the basinasal region (Plate VIII. fig. 2). 
The olfactory crura (Plate VII. fig. 11, 1) are now much nearer together; for the 
“perpendicular ethmoid” ( p.e .) has grown into a definite (posterior) part of the 
general internasal septum, whereas formerly (Plate VI. fig. 9, 1) it was merely so much 
cartilage as lay between the olfactory openings in the low, thick, transverse cranio-faeial 
wall. 
The sides of this barge- shaped skull are now well chondrified, each ascending lamina 
ending in a neat selvedge above (Plate VIII. fig. 1) ; the regions of this Avail are the 
“ ethmoid,” “ orbito-sphenoid,” and “ alisphenoid” (Plate VII. fig. 5, eth ., o.s., al.s.) ; the 
last of these has already received some bony deposit from the prootic centre (pro.). 
Sections made transversely through the skull show well the relation of the various 
parts. The first, through the anterior part of the “ hemisphere” (Plate VII. fig. 7), 
shows the cartilage of the ethmoidal region where it begins to be orbito-sphenoidal ; a 
bulging floor connects the trabecular thickenings ; and beneath this is the fore end of the 
parasphenoid (pa.s.) ; the ascending plate is first thin, then thick, and is then bevelled 
before it terminates above. 
The “ fontanelle’ has here the tips of the frontals (f.) partly enclosing it ; on the sides 
