The Growth-Rings on Herring Scales. 



259 



of minimum growth corresponding with the winter-season of the year and 

 thus affording an index of the age of the fish. 



At present the chief morphological evidence on the point rests upon the 

 observation that, in the majority of cases, scales taken from herring in 

 winter time have what is called a " clear edge " ; while those taken from fish 

 in summer appear striated right up to the extreme edge. It is thought, 

 therefore, that the " clear edge " of winter persists in more or less degree 

 and may be identified subsequently as the transparent ring. Xot only, 

 however, do many exceptions occur, but it is admitted, by those who have 

 the most thorough acquaintance with scale-reading, that it is not possible 

 to say by inspection whether a transparent ring is in process of formation 

 or no. 



It is the purpose of this communication to detail as shortly as possible 

 such additional evidence as we have been able to obtain with respect to the 

 structure and significance of these rings. 



Our observations fall under three heads : — 



(1) Comparison of dorsal and lateral scales taken from the same fish. 



(2) Comparison of the effect produced when the scale is viewed under 

 polarised light with that obtained under ordinary conditions. 



(3) The ring as seen in section. , 



Contrary to common belief, not all the scales of a herring are of the kind 

 described above. If scales be taken from the back, from the region lying 

 between the dorsal fin and the root of the tail, they will be seen to be marked 

 by concentric striae quite similar in their disposition to those of, say, the 

 salmon. For this reason, such scales indicate clearly a differential growth- 

 rate ; further, it can be shown beyond doubt that the limits of the close zones 

 of slow growth coincide in point of position with the transparent rings of 

 lateral scales taken from the same fish. It is fair, therefore, to assume that 

 the transparent ring on the lateral scale does in fact represent a period of 

 minimum growth. 



Before discussing our second observation, it should be remarked that the 

 scale of such a fish as the herring is made up of two distinct layers, an upper 

 layer lying unconformably upon a lower, which is itself built up of successive 

 lamellae added to the inner surface of the scale. 



Each lamella, as it is formed, is greater in area than any of those preceding 

 it ; and this process coupled with the simultaneous growth in extent of the 

 upper layer brings about the general enlargement of the scale. In section, 

 the upper layer may very clearly be seen lying, unconformably as it were, 

 upon a succession of " outcrops " of the lower. When viewed from above under 



