i 
428 MR. W. K. PARKER OK THE SKELETON 
large notochord {no.) enclosed in its thick fibrous sheath ; it is cut through along 
its axial fissure (Plate 23, figs. 4-7, nc.), and its apex reaches to the part where 
the posterior nasal canal ( p.n.c.) emerges from the skull cavity. 
Below the notochord the special, narrow, long food-pharynx ( plix .), is laid open; 
near the head this passage bends downwards under the nasal canal, and over the 
frimbriated shelf (see fig. 2, i.v.s.) the “intervelar shelf,” or fringed floor. Then, 
enclosed in connective tissue, we see the special branchial diverticulum of the 
pharynx, the branchial bronchus, perforated by seven erect, oval, valvular apertures or 
clefts, each of which opens into a branchial pouch (see fig. 2, i.h.ci'.). Below this large 
perforated tube, a cul de sac, another similar but smaller perforated tube, is seen; 
this is the great branchial artery (or aorta) [hr.a.), giving off its secondary branchial 
arteries to the pouches. Below these parts we see muscles and inter-muscular septa, 
the structure of which does not concern us now. In front of the structure just 
described, where the mucous membrane is lined with cells derived from the hypoblast, 
we see the large oral or buccal involution, the epithelium of which is epiblastic. 
This is divided into two regions, the labial, or region of the disk, and the proper 
mouth cavity (m.), expanding into a buccal space right and left of the vela and inter¬ 
velar valve. The first space is a low, highly ornate dome, pierced above to form the 
proper oral opening (see Plate 8, fig. 11), which is reduced to an arched fissure by 
the broad dentate end of the tongue. The oral cavity is very extensive, but it is 
partly taken up in front by a peculiar shelf, containing the great upper labial ( u.l L ), 
and, behind, its height is lessened by the pouched end of the posterior nasal canal 
(p.n.c.). That canal is seen to emerge from the cranial cavity in front of the 
notochord, and then to end between the notochord and the depressed part of the food- 
pharynx (phx.) ; it has a right and left diverticulum near its end. Narrowing in its 
intercranial tract, we see it widen again, under and in front of the nasal sac (na.), where 
it opens with the nasal opening (e.n.) ; its general form, in outline, is bracket-shaped. 
The skeletal parts displayed in this section are as follows:— 
The tegmen cranii ( t.cr.) over the junction of the mid and hind brain (C~., C 3 .); the 
nasal roof (nci.) and a sigmoid tract under and in front of the skull; this is intertra¬ 
becula behind, ethmoid in the middle, and cornutrabeculge (c.tr.) in front; the 
investing mass shows a little where it is cut through, over and under the apex ot 
the notochord. 
In the base of the mouth and throat the large basi-hyal (b.hy.) is halved, its soft and 
then its hard part, in front, is shown ; it reaches behind to the second branchial open¬ 
ing. The thickest part of the annular cartilage (l.l 1 .) is above, the narrowest is below, 
the thin arched cartilage above and behind the upper section is the great median 
upper labial (u.l 1 .). forming the skeleton of the shelf under the cornu trabecuke. Below 
the basi-hyal, the median distal mandibular ( m.d.m .), is seen close inside the proper 
chin, behind the fissure between the chin and the great sucking disk. The small 
