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ME. W. K. PA ETC EE, ON THE STEUCTUEE AND 
inner nostril , it will extend to the outer side as the external nostril ; like the cleft be- 
tween the first and second postorals (Plate III. fig. 10, 1), it is of small vertical extent. 
Third Stage. — Frog-tadpoles 5 lines long. 
This stage illustrates the period when the free gills are at their fullest development 
before the operculum has enclosed them; they have tertiary papillae at this time. 
In Plate IV. fig. 7, a Tadpole at this stage is shown with the skin removed from one 
side so as to expose the skull and face; the eye has been -dissected away so as to 
expose the first arch more perfectly ; but the nasal sac (ol.) and the now finished ear- 
sac are left in situ. All the chondrified parts, except the “investing mass” and the 
azygous basal elements of the face, are shown at once in this view, which is extremely 
instructive, and the meaning of which will be evident to the ichthyotomist at once. 
The facial arches have not changed their relative size so much as their shape and 
position ; transverse segmentation is now complete in the first and second postorals ; 
and secondary growths , which may be called “ connectives,” have now begun to bind 
together these primordially free rods. The cranial cavity has now nearly lost its 
bend upon the spinal tube (figs. 7 and 8); and the first three arches have followed in its 
ascent, so that they now form an acute instead of a right angle with the general axis of 
the Tadpole ; the gill-arches are still almost vertical. The first or preoral arch has now 
begun to hide itself beneath the membranous cranium, with which it will soon coalesce, 
and outside which it will set up the chondrifying process. Even externally it can be seen 
that the first and second rods are coalescing with each other above, where they lie close 
to each other ; below this part they diverge — not, however, to lose connexion ; for at the 
lower third a transverse band appears, a “connective” uniting the trabecular arch with 
the mandibular. This band, which had no existence in the last stage (Plate IV. fig. 1), 
is the first rudiment of the pterygo-palatine bar, so largely developed in the adult Frog. 
This “ connective” lies, of necessity, inside the temporal (crotaphite) muscle. But the 
mandibular pier sends upwards and forwards another leafy growth of cartilage, which 
embraces the temporal muscle from its outside; this is the “ orbitar process” ( orp .), a 
very characteristic larval batrachian structure. We can now see what becomes of the 
imperfectly open cleft between the first and second primordial arches : the space between 
the upper coalesced part and the pterygo-palatine connective becomes the “ subocular 
space,” whilst the space below and in front of the pterygo-palatine becomes open, first 
within and then on the outside , as the canal of the nasal sac, the openings being the in- 
ternal and external nares. The upper part of the first postoral arch may now be 
called the “ mandibular pier,” because the true mandible is now well developed ; this is 
Meckel’s cartilage (ink.) ; it is a short stout club, with a fossa for the condyle on the fore 
part of the lower end of the pier. It is not functional at present to a great degree, and 
turns upwards and inwards in thickness of the lower lip (see also fig. 8, mk.). The 
upper part of the mandibular pier, as far down as to the pterygo-palatine connective, 
may now take the name of the “ metapterygoid region,” whilst the rest of the bar is the 
