RATHEE ON THE DEVELOPMENT OE THE CRANIUM. 
247 
pears to consist of the dura mater and of some cellular tissue. Quite 
at the anterior end of the brain case the two bands unite, but coalesce 
only for a short space, and then separate at a not very acute angle. 
These anterior diverging parts, which belong to the face, have a con¬ 
siderable length, are about half as broad as they are long, and 
anteriorly are obtusely rounded off. At their sides and quite poste¬ 
riorly, where they pass off from the coalesced part of the trabeculae, 
are the nasal apertures and cavities. The anterior ends of both how¬ 
ever come into contact with the semilunar pieces of cartilage which 
constitute the upper beak of tadpoles. In front of the auditory 
capsules, the lateral walls of the cerebral capsule are slightly 
chondrified, but the entire upper wall of the brain case is mem¬ 
branaceous. 
As regards the further development of the skull, the cephalic 
part of the notochord persists until almost the end of larval life, but 
the investing mass, which immediately surrounds it, remains through¬ 
out life cartilaginous, so that a basi-occipital is never formed. 
On the other hand, the two processes which the mass has sent 
out close behind the auditory capsules (the exoccipitals) ossify 
long before the end of larval life. But far earlier than this happens, 
a thin cartilaginous plate of considerable length and moderate 
breadth is developed, for the most part close beneath the cellulo- 
membranous substance, which, as a delicate membrane, fills the space 
between the paired trabeculae cranii, and for a less extent, immediately 
under the anterior part of the cartilaginous plate, from which those 
processes are continued, (in part therefore beneath the investing 
mass of the notochord,) and at its posterior end sends out at right 
angles two short and broad arms. Soon after it appears it ossifies 
and then, as in osseous fishes, takes the place of the two sphenoidal 
bodies of the higher animals. Beichert believes that this plate is 
developed from the mucous membrane of the mouth, and that it 
has nothing to do with the sphenoid bones of the higher animals. 
(‘ Entw. Greschichte des Kopfes d. nackten Amphibien,’ p. 35.) But it is 
only loosely connected with the mucous layer, and more closely with 
the parts of the base of the cranium already mentioned which overlie 
it; further, in old larvae and in adult Frogs, the fibrous membrane of 
the cartilaginous and bony parts beside it (viz. of the occipital bone 
and of the trabeculae) is seen distinctly to be continued under it; and 
finally, the independence of this osseous plate from the mucous 
membrane is testified by the analogy of its formation with that of 
the pre-sphenoid of Beptiles and Birds. 
That the bony plate in question answers to the pre-sphenoid of 
the higher vertebrata is shown by its arising in a similar place and 
in a similar manner, as the latter, viz. : in the immediate neighbour¬ 
hood of the posterior part of the trabeculae cranii, and not by the 
ossification of any part of the investing mass of the notochord itself. 
Yet by applying itself to the under side of the persistently carti¬ 
laginous plate formed from the notochordal investment, and thus 
