. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. once formed, the calcareous additions whirh give origin to its base (c) gradually cease to be in exact apposition with the original protomorphic zone; and in proportion as the base of the spine extends, have we a wider and wider interval, occupied by the tissue of the enderon, between its upper surface and the under surface of the ecderon (/). Examin- ing it in the perfect state, then, it would appeal- that the spine is included in a sac of the en- deron ; and this appearance is very much strengthened if dilute hydrochlo
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. once formed, the calcareous additions whirh give origin to its base (c) gradually cease to be in exact apposition with the original protomorphic zone; and in proportion as the base of the spine extends, have we a wider and wider interval, occupied by the tissue of the enderon, between its upper surface and the under surface of the ecderon (/). Examin- ing it in the perfect state, then, it would appeal- that the spine is included in a sac of the en- deron ; and this appearance is very much strengthened if dilute hydrochloric acid be added, by which the enamel layer («) is dis- solved out, and the structureless membrane enclosing the spine rendered distinct; while its continuity with that structureless layer which bounds the enderon is at once obvious. From its development, however, it is clear that this is a simple appearance, and that the apparent sac results from the projection in- wards of the extremity of this truly ecderonic structure. In fact, inasmuch as the base of the spine grows like its shaft by continual ad- dition to its inner surface, while its apex is unquestionably an ecderonic structure, this base might be considered to be enveloped in an involution of the protomorphic plane of the ecderon (fig. 307. c). Now suppose such plates as these to have acquired their maximum in width and mini- mum in height; furthermore, imagine them to be so closely set in the skin that the posterior edge of one over-rides the anterior edge of the one next behind it, and we have the exact ar- rangement of the scales in the cycloid and ctenoid fish (fg. 309.).* Fig. It is particularly to be remarked, however, that the projecting body of the spine being Scale of the Roach (Leuciscus.') A, section; B, surface. * The flexible cycloid scale of the eel presents an exact parallel to the tooth-like placoid scale of the skate, except that it is flat instead of conical, and that, in the adult state, the
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