AS AN INDEX OF AGE. 53 



season. Thus by following the arrangement of the lines of growth on scales, 

 it is a simple matter to observe the starting-place of any year's growth by the 

 comparatively wide separation of the growth-lines at that portion of the scale, 

 and in this way the surfaces of scales appear mapped out V)y annual rings. 

 These annual rings supply us with an index as to the age of the fish, and may 

 be roughly compared to the rings in many trees. The annual rings in the 

 stems of trees are due to seasonal nutritive conditions, and the rings on the 

 scales of fishes are probably the result of seasonal environmental conditions, 

 such as food, temperature, etc. In more detail, the alternate occurrence of 

 comparatively rapid and slow areas of growth in scales is probably the result 

 of the variations in food, temperature, etc., which are associated with the 

 alternation of summer and winter. For example, the abundant supply of food 

 ([)lankton, etc.) during the warmer season of the year probably has much con- 

 nection with the comparatively rapid growth of the scale at that time as 

 compared with the slow increase during the colder season, when there is a 

 decrease in the supply of food. 



"These facts appear to possess both scientific and economic importance, since 

 they permit the extension to marine fishes of a new system of age-determina- 

 tion by means of these annual rings on scales, a system which has recently 

 been show'n and demonstrated by Dr. Hoffbauer for the carp. 



"I hope to illustrate clearly the mode of formation of annual rings in Gadoid 

 scales by the aid of the figure on the accompanying plate. 



"The figure [Plate II., Fig. 1, of the present paper] represents the scale of a 

 pollack, 28-5 centimetres (11| inches) in length, captured towards the end 

 of October. A minute translucent area (see Fig. 1, C) devoid of any lines is 

 situated towards the narrower and more internal end of the scale ; and around 

 this area, which is the first portion of the scale to be formed, are grouped 

 numerous excentric lines of growth similarly disposed to the excentric layers 

 in the starch grains of the potato. 



"The excentric lines of growth on this scale, however, are arranged in such a 

 manner (see figure) as to map out its surface into two main regions, namely, 

 an internal area, which is the entire growth of the first year, and an 

 external part, the summer growth of the second year. One understands how 

 these two areas appear so distinctly if one follows the lines of growth out- 

 wards from the translucent area to the broader and more external part of the 

 scale. One may firstly observe that there are nineteen lines comparatively 

 widely separated from one another, wdiich indicate the growth of the first 

 summer, and secondly, ten lines less widely separated, indicating growth of 

 the first winter. External to these there follows an area showing much more 

 widely separated lines of growth, which indicate the scale growth of the 

 second summer. 



"The difi'erence between the lines of growth formed during the second 

 summer and those of the preceding winter is so apparent as to clearly define 

 the termination of the first year's growth. The widely separated lines of the 

 second summer number nineteen, and as the pollack from which this scale was 

 taken was captured in October, it appears that in this scale the number of 

 lines formed during the second summer exactly agrees with the number 

 formed during the first summer." 



