Section 28. Turning Now to Figure 8,i., We Have a Side View of a

tadpole's skull. On the ventral side of the head is a series of vertical cartilaginous bars, the visceral arches supporting the walls of the tadpole's gill slits. The first of these is called 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.