TEGUMENTARY ORGANS. 



483 



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 

 layer of 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. 



Scale of Sturgeon. 



A, one of the detached tubercles highly magni- 

 fied ; B, the entire scale. 



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 minute papillae (Jig.3\Q.A,a), 

 which may be followed for some distance on 

 to the hard portion. Furthermore, it exhibits 

 scattered round spots, with projecting centres 

 of the same appearance as the ridges, and like 

 them feeling hard to the touch. 



If asection of the scale be made (^g.310. B), 

 its under surface will be found to have a conca- 

 vity corresponding with the convexity of the up- 

 per. If the section has passed through one of 

 the ridges, it is seen that the osseous tissue of 



the scale is of two kinds ; a superficial homoge- 

 neous-looking, dense, comparatively thin layer, 

 and a deep, thick, laminated portion. If 

 traced from the centre of the scale to its an- 

 terior circumference the superficial layer loses 

 its continuity, breaking up into conical bodies, 

 which are the sections of the detached calca- 

 reous spots mentioned above ; the deep layer 

 thins out, its lamina? gradually becoming 

 fewer, and leaving a soft membranous space 

 between their upper surface and the under 

 surface of these spots. In the centre of the 

 scale again, a series of rounded apertures are 

 seen in a tangential section, the sections of 

 canals which radiate through the scale and 

 become more numerous and wider towards 

 its margin. They are connected below with 

 vertical canals passing through the laminated 

 layer, and anteriorly they pass into the wide 

 membranous space above referred to. There 

 is no histological difference of any importance 

 in the structure of these two layers ; each is 

 composed of true bone with radiated cor- 

 puscles ; the upper being more dense and ho- 

 mogeneous, the lower less dense and lami- 

 nated. 



If a section be made through several of the 

 ridges of the upper surface, it will be seen that 

 they are entirely composed of the hard homo- 

 geneous osseous tissue. On their sides, how- 

 ever, and in the valleys between them, more 

 or less of soft integument remains, whose 

 pigment masses give the valleys a dotted ap- 

 pearance. On the other hand, a section of 

 one of the detached tubercles shows, except 

 in its consisting of osseous tissue only, that 

 it is identical with a single spine of the Skate 

 (j%. 3 1 0. A). It appears to me, therefore, that 

 there can be no doubt that the ganoid, over- 

 lapping scale of the sturgeon commences by 

 an isolated p^coid spine ; that other spines 

 are developed around this, and their bases 

 uniting, constitute a placoid scale, between 

 whose elevations little valleys, bridged over by 

 the soft integument, remain ; that to the base 

 of such a plate as this, continual additions of 

 osseous laminae are made, the radiating Haver- 

 sian canals being left between the first lamina? 

 and the superficial plate ; and finally that, 

 extending in size, the anterior face of this 

 complex scale becomes over-ridden by the 

 preceding one. Complicated as it may ap- 

 pear, it is obvious that all this structure 

 results from the continued endogenous growth 

 and union of the primary ecderonic calcareous 

 deposits, which constitute, as it were, so many 

 centres of ossification for the large scale. The 

 final structure, however, is (if we leave out of 

 consideration its histological character), to all 

 intents and purposes, that of a cycloid scale ; 

 and its mode of growth is identical with that 

 of the large cycloid scale described by Prof. 

 Williamson. 



The increase of the scale is concentric; 

 addition being made to its posterior, as well 

 as to its anterior edge and surface ; the only 

 difference being, that in the latter case the 

 development of the upper layer is less rapid 

 than that of the lower, while in the former 



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