DEVELOPMENT OF THE SKULL IN STURGEONS. 
149 
Section 4.—The next slice (Plate 13, fig. 1) contains the hinder half of the auditory 
involution (aq.v.), and below it the auditory capsule (an.) is seen, in the region of the 
anterior canal; there is some cartilage in the infero-lateral part of the wall of the cap¬ 
sule. This is behind the mid-brain, and the hind-brain (see figs. 1-3, C 3 .) is very 
large; large masses of ganglionic cells are seen belonging to the 5th, 7th, and 8th 
nerves (V., VII., VIII.). Outside, the lip of the spiracle (cl 1 .) is cut through, and 
below the brain the large notochord (no.) is shown near its apex. This lies in and 
between the moieties of the investing mass (iv.), which are nearly square—-just a little 
hollow above, and convex below. Below these parts the oral cavity is laid open ; 
right and left of it the thinner hind part of the pterygo-quadrate (pg.q.) is severed, 
and, below, the teeth of the lower lip (t., l.l .). 
Section 5.—Here the hinder part of the ganglionic mass belonging to the 7th and 
8th nerves (Plate 13, fig. 2, VII., VIII.), and some fibres of their cerebral roots are cut 
through; the spiracle (cl 1 '.) is here laid open ; above it a cartilaginous tract of the 
auditory wall is seen. The anterior and horizontal canals, and the vestibule ( a.s.c ., 
h.s.c., vb.) are laid open. The notochord (nc.) is enlarging, and is being enclosed in 
the square moieties of the investing mass (iv.) which lie somewhat under it. Below 
the mouth the mandibles (mk.) are laid bare throughout their entire length, on 
account of their almost directly transverse position, and over their articular hollow the 
articular knob (q.) of the pterygo-quadrate cartilage is shown in situ. The back of 
the lower lip (l.l.) is seen, whose toothed front face was shown in the last section. 
Section 6.—-Here, on each side of the narrowing hind-brain (Plate 13, fig. 3, C 3 .), 
the auditory capsules are cut through in their widest part, and both the arch and 
ampulla of the posterior canal (p.s.c.) are cut and laid open, as well as the deep 
saccular part of the vestibule (vb.). The infero-lateral shell of auditory cartilage is 
joining itself on to the more thin and outspread part of the investing mass (iv. —line 
too high); the notochord (nc.) is placed directly above the moieties of cartilage. The 
pharyngeal cavity is here very large, and in its walls we see the large, massive, arcuate 
epi-hyal element, undergoing segmentation into a long hyo-mandibular and a short 
symplectic piece (km., syi). Here the directly transverse position of the parts 
gives, in one section, the whole of the hyoid series, for the symplectic is seen to be 
followed, lower down, by the little inter-hyal (i.hy.), and that by the long cerato-hyal 
( c.hy .), which is giving off from its sub-triangular end the short hypo-hyal (h.hy.). 
This is the front face of the section, and behind and within the hyoid we see the 
lower half of the first branchial arch and the fore part of the basi-branchial bar 
(c.br 1 .); the branchial arch, like the hyoid, is becoming subdivided below, to form 
the hypo-branchial. The front part of the ganglionic mass of cells for the 9th and 
10th nerves (IX., X.) is cut across. 
Section 7.—These most successful and illustrative sections* display all that is sought 
for; here (Plate 13, fig. 4), on each side of the narrowing hind-brain (C 3 .), the 
* The sections were made by my son Mr. W. N". Parker. 
