628 
ME. W. K. PAEKEE ON THE STETJCTUEE AND 
(pd.) a large sinuous flap of cartilage growing from the front and side of the auditory 
capsule, and ending in a free “ foot ” ( t ., ty ., a.t.). 
All these parts have now to be described in detail. They are : — 1, parachordal ; 2, 
trabecular ; 3, mandibular ; 4, hyoid ; 5, auditory ; and 6, labial. 
The branchial arches will be described in the next stage. 
1. The parachordal bands enclose a large, long cone of notochord, with a rounded end, 
which is opposite the middle of the anterior semicircular canal ( a.s.c .). It has retreated 
considerably. Tracing the grooved tracts of newer cartilage, we see where these bands 
have united with the large auditory capsules ( au .), but not where they have passed into 
the older trabecular tracts (figs. 4, 5, iv., au., tr.). Below, the parachordals form a flap- 
shaped investment of the sides of the notochord ; and they expand to reach the auditory 
capsules, where the 9th and 10th nerves pass out (9, 10). 
Owing to the bulging of those capsules, these bands are pinched in the middle, and 
then they expand in front, where they are lost in the continuous mandibular and trabe- 
cular plates. Above, the parachordals are growing upwards, but they do not yet form 
a ring (fig. 4, e.o.). In front of the notochord and its investment, looking especially at the 
lower view (fig. 5), we see the cartilaginous floor growing forwards and outwards as Jive 
diverse leaves. The middle “leaf” is formed of two that have coalesced by their inner 
edges ; these are — 
2. The “ trabeculae, ” the first parts to chondrify. These confluent leafy bands extend 
from the notochord ( nc .) to the great, transverse, “ upper labial ” ( u.l .), fore and 
aft, and to athwart the “ subocular fenestrse ” ( s.o.f .) and pterygo-palatine plates (ppg.) 
on either side. They are more thoroughly seen above (fig. 4) than below (fig. 5). This 
is. on account of the somewhat lower position of the mandibular pedicle (pd.). Most 
of the median part is commissural, having been formed, later than the primary bars, 
beneath the huge pituitary space (see Plate 55. fig. 1). The actual relative width of the 
primary bars can still be seen where the “ cornua ” curve round in front of the nasal sac 
(e.n., i.n.), and behind the upper labial {u.l.). Hence we may infer that the large, gently 
concavo-convex plate (seen best below, on its convex face), up to the nasal sacs, is most 
of it the newer cartilage of the pituitary floor. On the sides, looking at the under 
surface (fig. 5), the trabecular boundaries of the old pituitary space have become leafy 
and hollow, elegantly, with an arcuate margin, widening up to their union with the 
pterygo-palatine bars (p.pg.). There they have become thick, and then, opposite their 
internasal region, they give off a pair of arms, like the arms of a sign-post. These arms 
(ppg.) end in the quadrate condyle (figs. 4 & 5, q.), which scarcely projects beyond the 
pterygo-palatine (compare Plates 55 & 56). 
From above we see that where the trabecular thickenings end there the banks of the 
median valley are not erect, but overhanging ; so that, in. front, the cranial sac is some- 
what enroofed with cartilage ; but all in front of that cavity, up to the nasal sacs (i.n.), 
is the ethmoidal region, which is almost as long as the cerebral. The ethmoidal part 
of the valley is now filled with a watery tissue ; but in the next stage (Plate 57. fig. 1) 
