TEGUMENTARY ORGANS 385 



to its anterior circumference the suJDerficial layer loses its continuity, 

 breaking up into conical bodies, which are the sections of the detached 

 calcareous spots mentioned above ; the deep layer thins out, its 

 laminae 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 corpuscles ; the upper 

 being more dense and homogeneous, the lower less dense and 

 laminated. 



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 

 homogeneous osseous tissue. On their sides, however, and in the 

 valleys between them, more or less of soft integument remains, whose 

 pigment masses give the valleys a dotted appearance. 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 {fig. 310. A). It appears to me, therefore, that 

 there can be no doubt that the ganoid, overlapping scale of the 

 sturgeon commences by an isolated placoid 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 laminse are made, the radiating 

 Haversian canals being left between the first laminse and the super- 

 ficial plate ; and finally that, extending in size, the anterior face of 

 this complex scale becomes over-ridden by the preceding one. Com- 

 plicated as it may appear, 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 



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