DEVELOPMENT OE THE SKULL OE THE COMMON FKOG. 
151 
backwards, and then as suddenly forwards : these curves are clearly seen in the large Tad- 
pole, the next stage. In this figure (10) it must be held in mind that these roots lie on 
a much higher plane than the amputated quadrates (q.). The figure must be compared 
with the lateral view (fig. 7) ; in this last the outer part of the metapterygoid is really 
the elboiv shown in fig. 10 ; out of sight in the lateral view, the secondary connective bar 
grows suddenly forwards as well as inwards. In this lower view the upper part of the 
cleft between the prestomal and first poststomal arches is shown ; it is a sigmoid tract 
of membrane (see also fig. 7, s.o.f.), the “ subocular fenestra.” The direction of the 
pterygo-palatine bar (ppd-) is downwards, outwards, and backwards ; its development 
is a measure of the distance travelled by the second from the first primordial arch — the 
lower end of the mandibular pier (the quadrate hinge) ultimately lying directly below 
the exoccipital, the pterygo-palatine bar becoming nearly as long as the very elongated 
mandibular ramus. In the vertical section (fig. 8) many parts are brought into view 
and shown in their relation, which cannot be so well seen in other aspects. In the large 
lips we see the upper and lower left labials ( u.l ., /./.), the end of the left “ trabecular 
cornu” under the prosencephalic region of the membranous cranium, the trabecular 
commissure ( tr.c .) in section lying obliquely in a fold of palatal mucous membrane ; the 
edge of the left moiety of the investing mass (i.v.) seen beneath the notochord (n.c.) and 
in the floor of the mouth cartilages are seen either as ends or sections ; these will be 
better understood by reference to horizontal views. 
The intestinal canal has now become ready for its functions, and the pharynx is narrow- 
ing where it is passing into the opening cardia. The heart is well formed in an ichthyic 
manner. In a horizontal section through the upper lip and labials (u.l) and at the 
lower third of the facial arches (fig. 11) w r e get a bird’s-eye view of the lower lip bulged 
into two rounded ridges by Meckel’s cartilage (mJc.), and of the tongue ( t.g .) bulged by the 
massive stylo-cerato-hyals ( hy .) ; the broad ramus of each side is seen in section further 
outwards. In front of this ramus is an oblique view, in section, of the quadrate region of 
the first poststomal arch ; and behind the hyoid is a deep fossa severing it from the first 
branchial (hr. 1) ; all the branchials (hr. 1-4) are seen to be flattened from before back- 
wards obliquely ; and the termination of their intervening clefts, a good distance from the 
mid line, is seen. In a section made lower down and more perfectly dissected (fig. 12) 
the lower part of the quadrate is followed by the massive hyoid, and, the soft parts of the 
tongue being dissected away, there is seen a pisiform keystone of soft cartilage, soft 
for weeks to come, being late in becoming hyaline ; this does not occupy the whole mesial 
space, but is only behind; this is the “basihyal” ( b.hy .), and is seen in section in fig. 8. 
Behind the basihyal (figs. 8 & 12, b.hy.) there is a large elegantly pyriform plate of 
cartilage forming a keystone to the first and second branchials ; it is the basibranchial 
bar ( b.br .), and contains two potential segments, the first and second of ichthyotomy. In 
the figure (12) the first branchial is seen to be becoming angular in section ; this will be 
understood afterwards ; the last two, seen from their lower extremity in fig. 8, are curved 
upwards towards the mid line, in conformity with the much narrowed pharynx. These 
