28 THE TERIODIC GROAVTH OF SCALES IN GADID.E 



deptlis of the skin. In tlie majority of scales the focus is carried backwards 

 a greater or less distance from the centre of the scale, for example, in the 

 perch, sole, brill, carp. In some cases the focus has been carried so far back- 

 wards that it is situated at the posterior border of the scales, as in several 

 species of gobies. It is much more rare to find the focus carried forward 

 from the centre of the figure ; this is met with in the scales of the tench 

 (Cyprinus tinea). Baudelot states that the scale increases at its periphery, 

 and that there is not any true growth at the focus by intussusception ; but 

 only a process of simple repair, which may modify the configuration of the 

 calcareous reliefs or cause their disappearance by transforming, for example, a 

 surface primitively covered with regular concentric ridges into a granular 

 surface with tubercles or with vermiform ridges. 



Agassiz and Vogt regarded the focus as the result of the wearing down of 

 the central portion of the scale. Peters successfully refuted this interpretation 

 by observing that the frictional or wearing-down process could not take place 

 owing to the fact that the scales are contained in pouches of the dermis, which 

 would protect them. In order to explain the existence of the focus, Baudelot 

 points out that scales frequently show zones with irregular ridges alternating 

 with zones with normal and regular concentric ridges, and he concludes that 

 the cause which produces this regularity or irregularity of the disposition of 

 the ridges is itself very unstable, and he holds that it is some such cause which 

 produces the focal region ; in short, this hypothesis supposes a change in the 

 mode of distribution of reliefs during successive epochs of the life. Baudelot 

 held, however, that he had not a sufficient number of facts either to confirm 

 or negative this hypothesis, and left the matter in abeyance. 



10. TJie tissues of scales. Scales are composed of two substances : {\) funda- 

 mental organic substance; (2) inorganic substance. The fundamental organic 

 substance belongs to the group of connective tissues (dermal) ; the inorganic 

 substance consists of calcareous concretions of phosphate and carbonate of 

 lime. The fundamental organic substance is more or less transparent and 

 homogeneous in appearance, and is readily broken up into folia which are com- 

 posed of elementary fibrils. By dissection or through the action of reagents, 

 such as soda or potash, it is easy to separate the component folia of scales from 

 one another. These folia are extremely thin, are superimposed the one upon 

 the other like the leaves of a book, which become smaller as they approach 

 the external face of the scale. The scale is more or less like a cone with a 

 large base, and in which plates or folia are piled the one on the top of the 

 other from base to summit. These folia separate from one another most 

 readily in the median portion of the scale, but not so readily at the periphery, 

 at which region, indeed, they adhere to one another so intimately that it 

 becomes difficult to isolate them without tearing them and getting fragments 

 of several adjoining folia. Isolated folia are somewhat transparent, flexible, 

 and membranous. They are not entirely homogeneous, as Avith a high power 

 they show in their thickness a fine striated appearance. At the perii)liery of 

 the lamellae, where rents have been made, the tissue shows itself decomposed 

 into fibrils or into bundles of fibrous tissue. The striae of adjoining lamellae 

 do not follow the same direction, but cross at angles to one another. At the 



