. The elasmobranch fishes . Fig. 33. A. Section through a light organ or photophore, Spinax niger. (From Johann.) B. Pigmentation of a photophore, Etmopterus. (From Oshima.) , blood sinus; gr., basal or germinative layer of epidermis; ir., iris; , lens cell; , light cell. As the papilla grows upward it raises the basal layer of the epidermis, making of it a cap, the enamel organ (). By continuous growth the cells of the enamel organ assume the high cubical type with their nuclei located toward the outer margin. The cells of the enamel organ form a layer of enamel over the tip o
. The elasmobranch fishes . Fig. 33. A. Section through a light organ or photophore, Spinax niger. (From Johann.) B. Pigmentation of a photophore, Etmopterus. (From Oshima.) , blood sinus; gr., basal or germinative layer of epidermis; ir., iris; , lens cell; , light cell. As the papilla grows upward it raises the basal layer of the epidermis, making of it a cap, the enamel organ (). By continuous growth the cells of the enamel organ assume the high cubical type with their nuclei located toward the outer margin. The cells of the enamel organ form a layer of enamel over the tip of the papilla; while the odontoblasts of the dermal papilla which lie most superfi- ciall}^ are the first to lay down dentine. The first layer covers the tip and sides of the papilla lying immediately under the thin layer of enamel. Then the odontoblasts which are located deeper send out processes around which den- tine is deposited. The canals thus formed for the processes themselves are the beginning of the dentinal canals and into them the protoplasmic processes of those odontoblasts lying still deeper will later enter as the formation of the dentine continues. Thus it is that the dentine produced from without inward becomes thickened, finally crowding the core of the papilla into narrow compass. When a scale like the one described above reaches the surface the epidermal layers are rubbed off from the tip, and the body of the scale then erupts. In a more mature embryo than the one here described many such scales erupt at about the same time and come to take up a definite arrangement in patterns in
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