th the corresponding diagrams
of the other types.

2. _The Skull of the Frog (and the vertebrate skull
generally)_

Section 22. We have already given a description of the mammalian
skull, and we have stated where the origin of the several bones was in
membrane, and where in cartilage; but a more complete
comprehension of the mammalian skull becomes possible with the
handling of a lower type. We propose now, first to give some short
account of the development and structure of the skull of the frog,
and then to show briefly how its development and adult arrangement
demonstrate the mammalian skull to be a fundamentally similar
structure, complicated and disguised by further development and
re-adjustment.

Section 23. Figure 1,I. Sheet 14, shows a dorsal view of a young
tadpole cranium; the brain has been removed, and it is seen that it
was supported simply upon two cartilaginous rods, the trabeculae
cranii (tr.c.). Behind these trabeculae comes the notochord (n.c.), and
around its anterior extremity is a paired tract of cartilage, the
parachordals (p.c.). These structures, underlying the skull, are all that
appear[s] at first of the brain box. In front, and separate from the
cranium, are the nasal organs (n.c.); the eyes lie laterally to the
trabeculae, and laterally to the parachordals are two tracts of cartilage
enclosing the internal ear, the otic capsules.

Section 24. Figure 1, II., is a more advanced, phase of the same
structures. The trabeculae have met in front and sent forward a
median (c.t.) and lateral parts (a.o.) to support the nasal organs. They
have also flattened, out very considerably, and have sent up walls on
either side of the brain to meet above it and form an incomplete roof
(t.) over it. The parachordals have similarly grown up round, the
hind-brain and formed a complete ring, the roof of which
is indicated, by b. Further, the otic capsules are fusing with the
brain-case. With certain differences of form these elements-- the
trabeculae, the parachordals, and the otic capsules, are also the first
formed structures of the mammalian cranium.

Section 25. In Figures 1,I. and II., there appears beneath the eye a
bar of cartilage (p.p.), the palato-pterygoid cartilage, which is also to
be seen from the side in Figures 8,I. and III. It will be learnt from these
latter that this bar is joined in front to the cranium behind the nasal
organ, and behind to the otic capsule. The cartilaginous ba