. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates. 40 COMPARATIVE ANATOMY is the enamel, which is formed as an excretion of the epidermic cells (Fig. 31), while the later formed mesodermal dentinal and bony portions become closely connected with the enamel secondarily. Thus the enamel is the first and originally the only hard substance of the placoid organ. In the Holocephali (Chimsera, Callorhynchus) a double row of placoids is developed along parts of the dorsal region in the embryo, but disappear in later stages: in the adult these organs apart from the spine on the an
. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates. 40 COMPARATIVE ANATOMY is the enamel, which is formed as an excretion of the epidermic cells (Fig. 31), while the later formed mesodermal dentinal and bony portions become closely connected with the enamel secondarily. Thus the enamel is the first and originally the only hard substance of the placoid organ. In the Holocephali (Chimsera, Callorhynchus) a double row of placoids is developed along parts of the dorsal region in the embryo, but disappear in later stages: in the adult these organs apart from the spine on the anterior margin of the dorsal fin, occur only on the claspers and frontal organ of the Teleostomes.—In these Fishes, ossifications in the derm to form bony scales takes place independently of any stimulus from. FIG. 31.—VERTICAL SECTION THROUGH THE SKIN OF AN EMBRYO SHARK. (From Gegenbaur's Comp. Anatomy.} C, derm ; c, c, c, d, layers of the derm ; E, epiderm ; e, its layer of columnar cells ; o, enamel layer ; p, dermal papilla. the epiderm. Thus the denticle, which in Elasmobranchs is the primary cause of the development of the basal plate, gradually disappears in ontogeny, and the latter is the only part of the placoid organ which remains, its independence being retained in the formation of bony skeletal substance in higher Vertebrates. In Lepidosteus, denticles are still developed in the skin but are quite transitory, and this primitive method of starting the formation of bony tissue is again met with in the Vertebrate series in connection with certain parts of the Amphibian skull; here certain bones (vomer, palatine, pterygoid, &c.) which originally served as supports for oral teeth, persist even if the teeth dis"- appear, as they have become an integral part of the facial skeleton. 1 In addition to the ordinary placoid scales, larger or smaller spines of a similar structure may become developed in connection with the dorsal tin, around the first cartilag
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