. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 92 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. be noted that most of the membrane bones of the skull (infra) belong in this categor}'. Here, too, ma)- be enumerated feathers, hair, horn, claws, etc., as well as cornifications of the skin of more limited distribution. Scales. — The most primitive t)-pe of this exoskeleton is found in the scales of the elasmobranchs. Here papillae of the derma (dentinal papillae"), arranged in qnincunx, push up into the epidermis, carr)'ing the basal layer of the latter before them. The exte
. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 92 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. be noted that most of the membrane bones of the skull (infra) belong in this categor}'. Here, too, ma)- be enumerated feathers, hair, horn, claws, etc., as well as cornifications of the skin of more limited distribution. Scales. — The most primitive t)-pe of this exoskeleton is found in the scales of the elasmobranchs. Here papillae of the derma (dentinal papillae"), arranged in qnincunx, push up into the epidermis, carr)'ing the basal layer of the latter before them. The external surface of each papilla and its base secretes a little plate of bone or dentine with a central spine; while the epi- dermis covering the papilla becomes converted into an enamel. Fig. 98. Developing scales of" dogfish (^AcatUhias), b, basal layer of ecto- derm; t. derma (corium}; d, dentine; e, enamel; eOy enamel organ: p, pulp. organ, the deeper face of which secretes a hard enamel cap upon the dentine base, the enamel being thickest upon the central spine. These scales are known as placoid scales, and in their development they show the closest similarit\- with teeth (see p. 19). In the ganoids (Lepidosteus') the early development is as in elasmobranchs, including the formation of plate, spines, and a rudimentary enamel cap. Later the spines and enamel cap dis- appear, while the outer side of the dentinal plate becomes cov- ered by a hard, smooth layer known as ganoin, which differs from enamel in that it arises from the derma. In the higher ganoids and in the teleosts, dentinal papillae are formed ; but the resulting scales are entirely of dermal origin, and, whether soft and flexible, hard and bon)-, show no differentiation into la vers. At first these scales are arranged in quincunx parallel to the. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illus
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