Section 3. the Skin of the Dog-Fish Is Closely Set With Pointed

tooth-like scales, the placoid scales, and these are continued over the lips into the mouth as teeth. Each scale consists of a base of true bone, with a little tubercle of a harder substance, dentine, capped by a still denser covering, the enamel. The enamel is derived from the outer layer of the embryonic dog-fish, the epiblast, which also gives rise to the epidermis; while the dentine and bony base arise in the underlying mesoblast, the dermis. A mammalian tooth has essentially the same structure: an outer coat of enamel, derived from epiblast, overlies a mass of dentine, resting on bone, but the dentine is excavated internally, to form a pulpcavity containing blood-vessels and nerves. Most land animals, however, have teeth only in their mouths, and have lost altogether the external teeth which constitute the armour of the dog-fish. Besides the teeth there perhaps remain relics of the placoid scales in the anatomy of the higher vertebrata, in the membrane bones. How placoid scales may have given rise to these structures will be understood by considering such a bone as the vomer of the frog. This bone lies on the roof of the frog's mouth, and bears a number of denticles, and altogether there is a very strong resemblance in it to a number of placoid scales the bony bases of which have become confluent. In the salamander, behind the teeth-bearing vomers comes a similar toothed parasphenoid bone. The same bone occurs in a corresponding position in the frog, but without teeth. In some tailed amphibians the vomers and splenials are known to arise by the fusion of small denticles. These facts seem to point to stages in the fusion of placoid bases, and their withdrawal from the surface to become incorporated with the cranial apparatus as membrane bones, a process entirely completed in the mammalian type.