'22 EVENINGS AT THE MICROSCOPE. 



radiating bands, but underlies the whole of the lined 

 parts. The concentric lines of the upper layer do not 

 now appear to be edges of successive plates, but irregular 

 canals running through the solid substance. This, how- 

 ever, is deceptive : for, by delicate focussing, we perceive 

 that each portion marked by these lines is really in a 

 different plane from the others, that the highest is at the 

 centre of radiation of the scale, and that each is succes- 

 sively lower till we reach the margin. But now, if with 

 very sharp scissors we cut one of these scales longitudi- 

 nally through the centre, and examine the cut edge, we 

 find that each of these lines forms a distinct ridge. On 

 the other hand, the under layer of clear substance is quite 

 smooth, and always a little exceeds the margin of the 

 concentrically lined portion. The clear substance that 

 tills the radiating slits agrees both in texture and level with 

 this lower layer, and is manifestly continuous with it. 



Hence, I think that, in these slit scales, the upper layer 

 is formed, as commonly believed, by successive deposits 

 from beneath ; but that, after a few have been deposited, 

 they begin to slit, probably by contraction in becoming 

 solid ; that the lower layer is formed after each upper one 

 is hardened, exceeding its length by a little, and filling 

 up the slit ; that this lower layer becomes the upper 

 layer of the next course, slitting, and turning up its 

 terminal edge as it hardens ; that then the lower layer 

 is deposited on this, filling up the slit as before ; and 

 that this process goes on as long as the fish lives. 



It is curious that, in the scales of the Pike, the por- 

 tions thus separated by slitting, instead of expanding 

 and leaving spaces to be filled up, actually close over 

 each other, the divided parts overlapping considerably, 

 as you may see in these specimens. The left-hand scale 

 (a) is from the back ; the central one (6), which has only a 

 deep narrow incision instead of a tube, is from the lateral 

 line ; and the third (c) is from the belly of the fish. 



