DEVELOPMENT OE THE SKULL OE THE COMMON EKOG. 
187 
never be misunderstood if its composite character be borne in mind ; the sensory organs, 
as well as the jaws, cheeks, and parts of the throat, must all be included in the term face. 
The Visceral ( Facial ) Arches. 
The Frog developes seven pairs of visceral arches : the seventh is not differentiated 
until a few days after hatching; and these simple, free, subarcuate descending rods 
undergo an amazing amount of metamorphosis. 
The last four, the gill-arches, are transitory, and their remains are of little importance 
in the adult ; the first three concentrate nearly all the interest upon themselves. 
The diagrammatic figures (Plate X. figs. 11-20) are intended to illustrate ten stages 
in the morphology of these three arches ; and as the auditory capsule enters into such 
remarkable relation with two, and especially with the last, it is figured also. 
The first arch is shown in dotted outline, the second in continuous outline, and the 
third is coloured. 
In the first stage (fig. 11) we see three curved clubbed rods, slenderest above, where 
they end in somewhat twisted points, which turn a little forwards. The first (1 tr.) is 
the “ trabecular ” rod ; it is recurved , and diverges from the next ; for here is the oral 
opening. The other two (2 mn ., 3 hy.) are very similar; but the foremost is the thicker 
of the two ; they are curved somewhat backwards as well as inwards, below. Here we 
have the rudiments of the first and second “ postoral bars,” or the mandibular and the 
hyoid. The auditory pouch is above and behind the third bar. 
Fig. 12 represents the second stage, a few days after hatchiug; in this already there 
are some changes to be noticed. 
The slight curve forwards of the narrowing upper part of the two foremost bars (1 tr ., 
2 mn.) has increased, the lower part of each has expanded, and the second has formed a 
small inturned bud, the rudiment of Meckel’s cartilage (rnk.). 
Another important change is the divergence, backwards, of the lower half of the third 
arch; and the approximation of the auditory sac to this arch above is noteworthy (3 hy., 
au.). 
Fig. 13 represents these arches in a Tadpole 5 lines in length ; here the changes have 
been sudden and great. 
The “ mesocephalic flexure ” is almost obliterated, and the first pair of bars (1 tr.) 
have also ascended ; but they have likewise applied themselves more accurately to the 
base of the membranous cranium, and have coalesced with each other in front of the 
pituitary body and with the second arch in two places. 
The second arch (2 mn.) has coalesced above with the investing mass and with the first 
arch ; it has diverged in its descent much more from the trabecular bar ; but at its lower 
third a connective bar has bound the two together (]r]X/.) ; outside this connective there has 
appeared a leafy flap of cartilage which encloses the temporal muscle. The free leaf of 
cartilage is the “ orbitar process” ( or.jp .), and the fixed secondary band is the first rudi- 
ment of the pterygo-palatine arcade ; this therefore is not in the Frog a primary arch. 
2 c 2 
