128 Part ITL.—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 oF 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 
