ed the hyoid arch (c.h.),
and the four following this, the first (br.1), second, third, and fourth
(br.4), branchial arches. Altogether there are four gill slits and between
the hyoid arch and the jaw arch, as it is called (= Meckel's cartilage +
the palato-pterygoid), is "an imperforate slit," which becomes the
ear-drum.* The frog no longer breathes by gills, but by lungs, and the
gills are lost, the gill slits closed, and the branchial arches
consequently much reduced. Figures 8, II., and 8, III., show stages in
this reduction. The hyoid arch becomes attached, to the otic capsule,
and its median ventral plate, including also the vestiges of the first,
second, and fourth branchial arches, is called the hyoid apparatus. In
Figure 5, the apparatus is seen from the side; c.h. is called the (right)
anterior cornu** of the hyoid. The function of the hyoid apparatus in
the frog is to furnish, a basis of attachment to the tongue muscles; it
remains cartilaginous, with the exception of the relic of one branchial
arch, which ossifies as the thyro-hyal (Figure 7 th.h.). It will be noted
that, as development proceeds, the angle of the jaw swings backward,
and the hyoid apparatus, shifts relatively forward. These changes of
position are indicated in Figure 8, III., by little arrow-heads.

* We may note here that, comparing the ear of the frog with that of the
rabbit, there is no external ear. There is, moreover, no bulla supporting
the middle ear, and the tympanic membrane stretches between the
squamosal in front and the anterior cornu of the hyoid behind. A
rod-like columella auris replaces the chain of ear ossicles, and may,
or may not, answer to the stapes alone, or even possibly to the entire
series. In the internal ear there is no cochlea, and the otic mass is
largely cartilaginous instead of entirely bony.

** Plural cornua.

Section 29. Before proceeding to the comparison of the mammalian
skull with this, we would strongly recommend the student thoroughly
to master this portion of the work, and in no way can he do this more
thoroughly and quickly than by taking a parboiled frog, picking off the
skin, muscle, and connective tissue from its skull, and making out the
various bones with the help of our diagrams.

Section 30. Figure 9 represents, in the most diagrammatic way, the
main changes in form of the essential constituents of the cranio-facial
apparatus, as we pass from the amphibian to the mammalian skull. F.
is