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Part III — Twenty-third Annual Report 



district, for instance, in the Western Baltic, where Jenkins studied, 

 but I have never stated that they use the same spawning grounds. 



Jenkins finds that the herring becomes sexually mature in its third 

 year. 



2. General Description of Lines op Growth. 



One of the chief objects of my observations was to test the question 

 how far the lines of growth in the skeletal structures of fishes were trust- 

 worthy indications of age, whether the annual increments of growth or 

 deposit could be definitely distinguished and counted in all cases. The 

 most direct and satisfactory basis for the assumption that the age of 

 individual fishes can be ascertained by inspection of lines of growth in 

 certain structures would be an extensive study of such lines in fish whose 

 age was known by direct evidence, but hitherto such study has not been 

 possible to any great extent. All I have been able to do is to ascertain 

 the age of specimens of different sizes as indicated by the lines and zones 

 of growth, and to compare the conclusions so reached with those to be 

 derived from other sources, such as the season in which the specimens 

 were collected, their size, and the evidence available concerning the 

 rate of growth from experiments with fish reared in captivity. 



Another object which was in view in the investigation was that of 

 discovering, as far as possible, the mode in which the lines of growth were 

 produced, what differences of structure caused the lines, and what was 

 the relation between the seasonal changes in external conditions and the 

 processes of growth taking place in the structures concerned. 



In the plaice successive more or less parallel lines and zones are visible 

 in the otoliths, in the scales, in the coracoid element of the pectoral 

 girdle, which consists of calcified cartilage, and the surfaces of the verte- 

 bral centra bounding the conical depressions in their anterior and posterior 

 faces. 



The otoliths consist of a number of thin layers deposited one over the 

 other around a common centre. The structure may be described as a 

 concentric stratification, and, apparently, when once deposited a layer 

 undergoes no subsequent change. The otoliths are thin and flat, but 

 one surface is more convex than the other, and this more convex surface 

 is in the natural position within the ear-capsule directed inwards and 

 the flat surface outwards. I find the most convenient way to extract the 

 otoliths is to split the skull with a knife from behind forwards, the 

 ear-capsules being then exposed, as they are not separated from the 

 cranial cavity by bone. The otoliths have a longer and a shorter 

 diameter, and along the direction of the longer diameter there is a groove 

 on the central part of the convex side. They appear to be formed as 

 concretions excreted by the epithelium lining the sacculus of the 

 auditory vesicle. 



Examined in water when freshly removed from the skull of the fish, 

 the otolith exhibits both concentric and radiating lines, so that its 

 structure resembles that of a scale, but the mode of formation is different, 

 the otolith being formed externally to the epithelium of the auditory 

 sac, which is derived originally from the epidermis (epiblast), while the 

 scale is formed within the derma (mesoblast). At first sight it might be 

 supposed that the successive deposits were formed only at the edge of the 

 otolith, but by examining a transverse slice of the object cut roughly 

 with a knife, it is seen that each successive layer extends over the whole 

 surface, but is exceedingly thin on the two flat surfaces and thicker at 

 the edge. The structure is such as would be produced if a sphere com 

 posed of concentric uniform layers of plastic material were very much 



