148 
MR. W. K. PARKER OK THE STRUCTURE AND 
The cerato-hyal element is a flattish rod, pinched in the middle, and dilated at both 
ends; its upper end is articulated to the postero-inferior face of the rudimentary 
symplectic by means of a small intercalary nucleus of cartilage—the inter-hyal ( i.hy .). 
The lower end of the bar is nearly segmented off as a short semi-oval hypo-hyal ( h.hy.). 
The branchial arches ( br .) are about half the size of the hyoid—supposing it to have 
been in one large bar; there are five of them, and they decrease in size from before, back¬ 
wards. The top of each is semi-distinct from the rest as a pharyngo-branchial {jp.br.); 
that part is turned inwards, the rest of the arch is curved outwards, and turns inwards 
and forwards, below, where it joins the basi-branchial bar (Plate 13, fig. 4, b.br.). 
Second Stage (continued).— Transversely-vevtical and vertical sections of the head of 
a larva of Acipenser ruthenus, 9J millims. in length. 
The transverse sections (Plate 12, figs. 1-3, and Plate 13, figs. 1-5) will further 
illustrate the structure of the visceral arches, and also show the state of the basis-cranii 
at this stage. 
Section 1.—The first section (Plate 12, fig. 10) is through the fore-brain (C 1 .), and the 
fore part of the mid-brain (C 2 .), the eye-balls (e.), and the barbels (bbl) ; here the only 
cartilaginous tracts are the trabeculae {tr.), which are sub-oval in section, obliquely 
placed, and separated by a distance equal to their own width; they are sub-concave 
on their upper surface, where they lie under the thalamencephalon as it narrows to the 
infundibulum. 
Section 2.—The rounded ends of the trabeculae are free for a very short distance 
in the extremely short head of this larva, which has at present no rudiment of the 
huge pre-cranial rostrum afterwards to be developed. In this section (Plate 12, fig. 11) 
the trabeculae are wider, flatter, and are in close contact with each other; they lie here 
under the infundibulum (C 1 .) which is covered now directly by the mid-brain (C 2 .). 
Here the fore part of the oral cavity is cut through, so that the front row of teeth (t.) 
are seen, and on each side the outer barbels {bb.) are just in view; this section is 
behind the eye-ball. Above the mouth two oval masses of cartilage, more solid than 
the trabeculae, are seen; these are the pterygo-quadrate bars {p>g.q.) at their fore end. 
Section 3.—The next (Plate 12, fig. 12) is close in front of the auditory capsules, 
through the widest part of the mid-brain (C 2 .) where it joins the hind-brain, in the 
middle of the mouth, and the huge Gasserian ganglia (V.), whose cerebral roots are 
brought into view. Here the trabeculae {tr.) are in close contact, are wider than in the 
last section, and they curve well round the base of the hind part of the infundibulum 
{inf.). 
Here the p terygo-quadrate cartilages (fig.q-) are very solid and curved, and have a 
sharp edge lying over tlie cc adductor mandibulae ” muscle ( ad.m .), (see also Salensky, 
op. cit., plate 18, fig. 165, ad.m.) ; these muscles lie outside the concave face of these 
oblique cartilages; the front teeth and lip are still seen. Above the front face of the 
auditory capsules (here, by mistake, lettered e), the auditory involutions (aq.v.) are 
seen, 
