160 
MB. W. Iv. PARKER ON THE STRUCTURE AND 
All trace of the distinctness of the auditory capsules and the investing mass is now 
lost, and the occipital ring is everywhere confluent with these organs. On the left side 
there is a section of the ampulla of the horizontal canal, and the crown of its arch is also 
shown (. h.sc .). Above and mesiad of this is part of the posterior canal (p.sc.). The left 
“ fenestra ovalis” is laid open ( fs.o .), and the right stapes (st.) shown in section. 
The form of the “ tegmen tympani” (t.ty.) is shown on the left side of the figure 
(above the stapes). The walls of the periotic capsule are very variable in thickness, the 
inner being thinnest ; this is only apparently separate from the “ investing mass,” the 
section being made through the “ meatus interims.” 
The oval section of the gelatinous notochord is seen to be invested below by the basal 
cartilage, but it is naked above (sec also fig. 6“, nc., b.o.). A posterior view of the same 
slice (fig. 7) passes through the epiotic region (cp.), and lays bare the hinder part of the 
arch and the ampulla of the posterior canal (p.sc.). The obliquity of the occipital ring 
of cartilage is such that this section, although passing through the periotic mass, is yet 
behind the occipital roof. 
The section has been made in front of the interspace for the “ vagus ” nerve ; but the 
glosso-pharyngeal (8") is seen to pierce the cartilage at the junction of the auditory cap- 
sule with the investing mass. This cartilage now entirely surrounds the notochord (see 
also fig. 7", b.o., n.c.). The oral cavity (to.) is now much narrowed, and is becoming 
the oesophagus. 
Fifth Stage. — Tadpoles with hind legs reaching to end of tail. 
Before the fore legs of the larval Frog appear, and whilst the tail is still of undimi- 
nished size, great changes take place in the actual and relative size of the parts com- 
posing the skull and face. 
The cells of the upper and lower labials (Plate VII. fig. 1, u.l ., l.l.) have begun to 
pass, by proliferation, into fibrous tissue ; the horns of the trabeculse cranii ( c.tr .) are 
becoming more expanded and adze-shaped, and their inner edges are growing mesiad 
to form the nasal floor. The upturned superior edge of the interorbital portion of the 
trabecula?, is growing further upwards into the side of the membranous brain-sac ; and 
the same upgrowth of cartilage has appeared over the auditory sac in the pterotic 
region ( p.t.o .). 
The trabeculoe (Plate VII. fig. 2, tr.c.) are still distinct to within a short distance of 
the commissure, which is now more extended antero-posteriorly (fig. 1, eth.). 
The nasal sacs (ok), approaching the mid line, lie on the trabecula? ; at present they 
are membranous, but they already have a floor formed by the trabecular horns, and a 
roof will soon be developed. 
Anteriorly the nasal sacs are only separated by soft, nascent tissue ; but there is a 
dividing wall behind ; this is the rudimentary “ septum nasi,” which has arisen like a 
rostral outgrowth from the middle of the rudimentary ethmoid (Plate VII. fig. 2, and 
Plate VI. figs. 9, 10, eth., s.n.). 
