. Text-book of embryology. Embryology. 322 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. the epidermis. Presently this begins to bulge upwards like a dome into the epidermis. The epidermal cells immediately bounding this little dermal elevation take on a columnar shape: they constitute the enamel epithelium (Fig. 160, e). The (dermal) cells on the surface of the dome or papilla, immediately underlying the enamel epithelium, also become distinct and form a definite layer of odonto- blasts. The hard substance of the scale makes its appearance, as a cone of dentine fitting over the surface of the derma
. Text-book of embryology. Embryology. 322 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. the epidermis. Presently this begins to bulge upwards like a dome into the epidermis. The epidermal cells immediately bounding this little dermal elevation take on a columnar shape: they constitute the enamel epithelium (Fig. 160, e). The (dermal) cells on the surface of the dome or papilla, immediately underlying the enamel epithelium, also become distinct and form a definite layer of odonto- blasts. The hard substance of the scale makes its appearance, as a cone of dentine fitting over the surface of the dermal papilla arid in turn ensheatbed by the enamel epithelium. The dentine cone, which usually becomes directed tail wards, gradually thickens, encroaching upon the dermal papilla or pulp which it surrounds. It lies E. Fig. 160.—Longitudinal vertical section through the skin of an embryonic Shark to show a developing placoid scale. (Prom Balfour's Embryology : figure by Gegenbaur after Hertwig, 1874.) E, epidermis ; e, enamel epithelium ; o, enamel; p, dermal papilla. immediately outside the odontoblasts and as it increases in thickness the outer portion of some of the odontoblasts persists as a fine thread of cytoplasm extending out through the substance of the dentine, so that when dried the dentine is seen to be traversed by innumerable fine slightly diverging canals each of which contained a protoplasmic thread. The hard material of the dentine is commonly regarded as calcified matrix but there is evidence which points rather to its being formed of modified cell cytoplasm. This point will be returned to in connexion with the development of the teeth. Towards the surface of the cone the calcified substance changes its character. It becomes extremely hard (consisting of very dense calcium carbonate), transparent and highly refracting, and the terminal branches of the tubules within it are reduced to an extreme degree of fineness. This outer layer is commonly known as enamel. I
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