622 
ME. W. K. PAEKEE ON THE STETJCTUEE AND 
shape of the fenestra, so is that of the 'plug which fills it (st.). The tissue of the 
walls of the capsule have long been hyaline cartilage (see figs. 1, 2, 3, au .); the plug 
(figs. 5 & 5“, st.) is still composed of granular indifferent tissue, very consistent, but 
not cartilaginous ; it will be cartilage when the Tadpole is the tenth of an inch longer, 
or in two or three days ; for in Tadpoles one third longer than this stage (1 inch 
long) it is very solid cartilage. In seeking to settle the dispute as to the nature of this 
plug, it may be remarked that it lies at the bottom of the periotic fossa, and that its 
outer surface is scarcely flush with the periotic wall around it, and that its inner face is 
in immediate contact with the membranous labyrinth. Now the upper segment of the 
“ columella,” which is chondrified two months or more afterwards, does fit into the 
depressed part or fossa (see “Frog’s Skull,” plate vii. fig. 16, and plate viii. fig. 9); the 
interstapedial segment ( it.st .) lies in the fore part of the stapedial fossa ( st.f. ), but the 
stapes itself covers the fenestra ovalis. When it is removed, the otoconial masses ( ot .) 
shine through the fenestra ( fs.o .). 
I shall next describe the tardy-growing antero-superior segment, which attaches itself 
to the periotic plug or stapes, and whilst homologous in its nature with the fish’s 
“ hyomandibular,” becomes, by specializing change, in time and in form, the auditory 
columella of a high Amphibian *. 
Skull of Bufo vulgaris. — Fifth Stage. Young Toads 5 lines long. 
These observations have been made to show the relation of the first and second post- 
oral arches to each other and to the auditory sac. My results correspond exactly with 
what Professor Huxley obtained first, in observing Bana temporaria. Seen from within 
(Plate 55. figs. 6, 7), with the basis cranii somewhat tilted up, the ear-capsule is found 
to be quite confluent with the parachordal cartilage (iv.) ; and now the tegmen tympani 
(t.ty.) is very large, and under its shadow we see the large, oval, plano-convex stapes, 
fitting into the recess which is open at its fundus behind, as the fenestra ovalis. The 
vestibular floor is gently convex, and is of less extent than the upper surface of the 
capsule, where the semicircular canals are imbedded; it retires most laterally, on 
account of the tegmen, but in front also, where the pedicle of the mandible runs (pd.), 
and also behind, where the 9th and 10th nerves (9, 10) pass out. The pedicle is much 
narrower, and clings close to the fore face of the capsule, as it passes into the tra- 
becula ( tr .), beneath the 5th nerve (5). But if the upper part of the pedicle is thin and 
narrow, its lower part is thick and broad ; and it is the great distinctness of this tract of 
cartilage, and the special facet or condyle upon it (c.pd.), which beguiled me in my early 
* Whilst carefully expressing my indebtedness to Professor Huxley for taking up this subject, and seizing 
by instinct, as it were, upon the erroneous parts of my description, I yet bold, most tenaciously, to my old 
view as to the homologies of the parts; namely, that the “ stapes” is a periotic, and the whole, complex “ colu- 
mella” a Jiyoiclean element. The fact that the stapes is formed out of the already chondrified periotic wall in 
the “ TJrodela,” as is most plainly seen in young Axolotls 2\ inches long, settles the stapedian homology; 
that the columella of the Batrachia is the homologue of the hyomandibular of Proteus , I have not the shadow 
of a doubt. 
