DEVELOPMENT OE THE SKULL OF THE COMMON FROG. 
179 
rest, separated by the free end of the perpendicular ethmoid ; this part is a feebly cari- 
nate projection, and sends backwards nothing that can be called a “crista galli.” The 
nerve-outlet is single on each side ; but this passage answers to the slit on each side the 
“crista galli” in the Mammalia, which in them is bridged over by many bars of bone, 
and thus converted into a series of foramina. 
The ethmoid is cranial as well as axio- facial, and just so much room as the “ rliinen- 
cephala ” need belongs to it by right, so that the “ girdle-bone ” is not the absolute 
measure of this region ; it does not reach far enough in the young of the Common Frog, 
nor in the adult of some species, whilst in others it trespasses on the surrounding terri- 
tories when they are unprovided with their own bony centres. 
Here, indeed, the “ rhinencephalic lobes ” do not reach so far back as the ethmoidal 
walls would indicate ; for these are partly built upon orbito-sphenoidal ground. There 
is scarcely any tendency to the production of a subcranial keel as a continuation back- 
wards of the perpendicular ethmoid ; but the basis cranii is flat and shark-like, only 
modified by a gentle bulging below (Plate IX. fig. 9). Where the basal part of the 
ethmoid joins the presphenoid its margin is gently concave; it is more scooped above, 
where the bony matter first began ; for here it forms the front margin of the great fon- 
tanelle (fo.). 
To sum up the characters of this simple ethmoid, let it be noted that the “ rhinen- 
cephalic fossce,” the roof upon which the frontals and, in most types, the top of the nasals 
rest, the floor , which in many types of Vertebrata joins a similar presphenoid behind, 
and the “ pars perpendicularis ” — all these are formed from one bony centre in the back 
wall of the nasal cavities, and the front end of the cranium. The cartilaginous founda- 
tions of this structure were laid in that part of the “ trabeculae ” which was the first to 
coalesce, in front of the original pituitary space, and from which coalesced portion there 
grew, at a very early period, a transverse wall separating the brain from the nasal sacs 
(Plate V. figs? 1, 2, 3). 
With regard to the succeeding regions, which have received anthropotomical names 
from the well-known bony centres, these must be surveyed from strictly neural land- 
marks, and not measured by the very indefinite bony tracts. The anterior sphenoidal 
region extends from an ideal line drawn circularly round the cranium outside the 
posterior margin of the “ rhinencephala ” to the “foramen opticum” (2). A section 
through the middle of this region shows the large orbital alee, “ orbito-sphenoids,” as 
a rather thin lamina of hyaline cartilage, which ends by a sharp edge above (Plate IX. 
fig. 9, os.). 
These alee pass into the “ presphenoidal ” region below, which is somewhat thicker 
where it bulges down upon its undergirding bone, the “ parasphenoid ” (p-s., pa.s.) ; 
these alee, thus united below, enclose the “ prosencephalon ” (C 1 ), the roof being strongly 
finished by the frontals, which send downwards rudimentary orbital plates. The anterior 
margin of the orbital alee has been ossified by the ethmoid ; for the rest there is only 
incipient “ endostosis ” below, in the presphenoidal region ; and this is not divided off 
2 b 2 
