50 THE PERIODIC GROWTH OF SCALES IN GADIDyE 



found in the arrangement of the concentric lines on the scale surface. The 

 best way to observe the arrangement of those lines is to begin at the centre of 

 growth, and to pass straight outwards to the median border of the anterior 

 area. The first lines round the centre of growth are rather irregular and 

 interrupted, and are comparatively widely separated from one another ; then 

 follow more regular lines, which lie close to one another until they run into a 

 bordering zone appearing somewhat darker. This zone marks the end of the 

 first year's growth. In the second zone (the second year's growth) the arrange- 

 ment of the concentric lines shows a repetition of that occurring in the first 

 year, namely, firstly, irregular lines comparatively separated from one another ; 

 secondly, more regular lines with little separation between them. In scales of 

 carp observed at the end of the third summer, a third zone shows itself similar 

 in general arrangement to the last. " The number of concentric lines within 

 a year's zone is, in individual scales from a scale row of the same carp, running, 

 for example, close above or beneath the lateral line, approximately the same. 

 Their number only decreases at those places where the scales themselves 

 become smaller, as at the gill-slit and at the tail; the number is also only 

 subject to slight fluctuations in corresponding scales on the right and left sides 

 of the same individual fish ; in corresponding scales from different individuals 

 it can, however, vary considerably, according to more or less intensive growth 

 within a year." 



He gives a statistical table to bring out these points, in which he shows the 

 number of concentric lines in individual scales in the row of scales dorsal to 

 and above the lateral line in the direction from the gill-slit to the tail. The 

 structure described above is seen in all normally formed scales, that is to say, 

 from carp living under favourable and natural conditions. lie, however, also 

 considers less favourable conditions. The fish's growth may have an irregular 

 course, it may grow faster or slower. For example, Avhat influence has illness, 

 or want of food, or both of these, for one or several years upon the method of 

 age-determination. 



But, " in this case, the structure of the scale does not leave us in a difficulty ; 

 on the contrary, we gain from it, in a manner, a self-registering, infallible con- 

 trol over the mode of life of its bearer." He takes a case to show this point. 

 He commences with the most unfavourable case : the case of a carp in its 

 third summer which had grown slowly all its life in consequence of less food. 

 This mode of life showed itself in the arrangement of concentric lines at an 

 equal distance from one another within a period of growth. As a result of 

 this, the border separating one year from another becomes more indistinct. 

 As a rule, one sees a divergence of the concentric lines at the age border where 

 the posterior area meets the anterior area at the right and left of the concentric 

 lines. If one follows this divergence towards the front margin of the anterior 

 field, then the separation area between one year and another becomes more 

 distinctly marked out than one had hitherto supposed, or rather observed. 

 Besides this, the radial lines also aid one in age-determination. It is frequently 

 the case that at the border between one zone and another, either several radii 

 of the i^revious year's zone end, or new radii of the succeeding year's zone 

 begin. Lastly, the total number of concentric lines is a sure way of dispelling 

 all further doubts on the subject. 



