20 THE PERIODIC GROWTH OF SCALES IX GADID.E 



one from the other, lose their regularit}', enlarge at certain points, or become 

 covered by tul)ercular projections (carp and other Cyprinids). 



In some fishes these ridges assume a peculiar mode of orientation. Instead 

 of following a course parallel to the contour of the scale, they take a direction 

 more or less perpendicular to this line, remaining, however, parallel to one 

 another (Alepocephalus, herring, shad). The particular disposition observed in 

 the herring, etc., is not an isolated fact, but the expression of a more or less 

 general fact which appears in various degrees. The number of concentric 

 ridges is not the same in the different regions of the scale. It is usuall}' much 

 greater in the anterior than in the lateral field, and in the lateral than in the 

 posterior field (perch, pike, minnow); this fact helps to prove that all the 

 ridges do not originate round the circumference of the scale. The number of 

 ridges may show the greatest variations in scales of the same fish ; the 

 number appears to be in proportion to the extent of the scale. Thus in large 

 scales from the flanks the ridges are relatively numerous, in very small and 

 rudimentary scales from other regions of the body (caudal fin, opercular 

 region) these ridges are extremely reduced in number. " Variations in the 

 number of ridges are not usualli/ great in scales front the same region. In 

 fishes of the same species, hut of different age, the numher of ridges increases 

 2n'oportio7iatelij with age, and consequently also loitli the dimensions of scales." 

 It ^■^.■ easg to verify this fact hy comparing scales from the sa)ne region in fish of 

 very different size. 



From this point of view, Baudelot made observations on scales of the pike, 

 perch, and minnow to determine the differences in number from simple to 

 double, triple, quadruple, and so onwards. 



New ridges are formed successively at a very sliglit distance from the 

 border of the scale by a partial calcification of the external layer. This 

 calcification shows itself firstly as a simple track of calcareous molecules in 

 the membranous zone which exists at the margin of the scale. This track of 

 molecules represents a calcigenous centre round which the calcareous substance 

 accumulates. 



From the thickening of this calcareous track there results firstly a slight 

 projecting part, wliich in raising itself soon constitutes a ridge. This enlarges 

 little by little at its base by the addition of calcareous molecules and finally 

 unites with adjoining ridges, so as to form a continuous calcareous invest- 

 ment on the surface of the scale. This mode of formation of ridges may be 

 easily followed in the scales of the sole, in the membranous zone which 

 constitutes the border of each of the lateral fields ; and it is also obvious in 

 the scales of many other types of fishes. 



The ridges of the scale surface examined under a very high power show 

 their free borders to be sometimes smooth, but in other cases crenated in such a 

 manner as to present fine denticulations. These denticulations may be seen in 

 the scales of the mullet, perch, and burbot, but the asperities are not uniformly 

 present on all the ridges of the same scale, and they may be completely absent 

 in the marginal ridges. In many scales (burbot, mullet) the concentric ridges 

 aopear to offer a marked inclination towards the centre of the scale. This 

 inclination shows itself by a more or less pronounced difference in the degree 

 of obliquity of the two planes corresponding to the two oi)posed faces of the 



