Hermann Mayer and Leydig have shown (and the fact is readily verifiable) that the scales and spines of the Plagiostome fishes are formed by the gradual deposit of calcareous matter in processes of the integument, which are at first coated by the ordinary cellular ecderon. These diverticula, in fact, originally resemble other papillm of the skin, and like them, are bounded by a structureless proto morphic layer, marking the boundary between the cellular ecderon and the enderon.
When the formation of the placoid scale commences, however, instead of the successive division and multiplication of the endoplasts and the cellulation of the periplast of the ec deron, which before went on, a deposit of cal careous matter takes place at the boundary line, and the structureless band remains as structureless or " basement" membrane, in vesting the future spine. The deposit in creases until the enderonic pulp occupies but a very small space, or even completely disap pears, and the spine projects as a cylindrical or conical tubercle. When it has attained its full length, the deposit does not cease ; new calcareous matter is continually added to its inner extremity, but rather in the direction of breadth than of length, so that, eventually, an irregular broad plate is formed with the spine projecting from its outer surface (fig. 308.).
It is particularly to be remarked, however, that the projecting body of the spine being once formed, the calcareous additions which give origin to its base (c) gradually ceuse 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 (f). Examin ing it in the perfect state, then, it would appear 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 (a) 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 (t. 309.).* A careful study of the scales of that remark able animal the Sturgeon, which exhibits in this, as in so many other characters, its inter mediate position between Teleostian and Pla giostome fishes, appears to me to throw still further light upon the difficulties .of scale development.
The scales of the sturgeon are large, slightly convex, rhomboidal plates, set obliquely in the skin, so that, while the posterior two-thirds of their surface are bare and hard, the anterior third becomes gradually softer from the pro longation of the integument over it. The posterior surface continues hard up to its sharp edge, but it is supported below by a soft thick layerof integument, which passes on to the an terior soft coat of the scale behind, and thus masks the real overlapping of this scale by the posterior edge of that which precedes it ( fig. 310. s).
The surface of the scale is shining and glassy. It is marked by a median ridge, whence it shelves upon each side, and by an elegant sculpturing produced by raised, hard ridges of the same nature, which radiate from the margins centrally, for about a fourth of the semi-diameter of the scale. In the region within this zone, the ridges gradually lose their regularity, the radiating lines anastomo sing with one another and forming an elegant polygonal network. The soft surface of the in tegument of the anterior portion of the scale, is raised into many min ute (jig. 310. A, a), which may be followed for some distance on to the hard portion. Furthermore, it exhibits scattered round spots, vrith projecting centres of the same appearance as the ridges, and like them feeling hard to the touch.