L32 



Part III. — Twenty-third Annual Report 



lines of greater opacity appearing white by reflected light, and these lines 

 are arranged in bands separated from each other by bands of darker, 

 more transparent bone. Each of these bands may indicate a year's 

 growth, being the summer growth separated from that of the next 

 summer by a band where there is less calcareous matter. But it is 

 difficult to decide exactly how many such annual zones are present. 

 Even when one or two of the outer zones seem distinct, the number of 

 the central earlier zones cannot be distinguished with certainty. One 

 may count three at one time, and at the next attempt there seem to be 

 four or five, and the total number always remains doubtful and uncertain. 



The otolith in cod and other Gadidae is large and opaque, and by 

 examining it as a whole nothing can be ascertained of its internal 

 structure. It is therefore, according to Heincke, useless for the purpose 

 of determination of the age of the fish. I have found, however, that the 

 successive laminae of which it is composed can be seen quite distinctly 

 in transverse slices simply cut from the central region of the otolith with 

 a scalpel. Such slices are, of course, rather thick, and their surfaces are 

 rough and irregular. Nevertheless, when they are placed in water in a 

 watch-glass and examined with a low-power objective, they are sufficiently 

 transparent to show the successive laminae of which the otolith is 

 composed, and the laminae in certain zones being much more opaque than 

 in the zones between these, the whole section is distinctly divided into 

 regions which I believe to indicate the annual increments, and which, 

 therefore, show the age of the fish. 



The otolith {i.e. the sagitta or largest otolith) of the cod is somewhat 

 elliptical in outline, with rather pointed ends, and two surfaces, one 

 convex and rather smooth, the other concave and more irregular. The 

 convex surface is turned inwards, i.e. towards the brain, and somewhat 

 downwards, the concave outwards and upwards. The convex surface is 

 marked by a shallow longitudinal groove, into which fits the ridge of 

 sensory epithelium, called the macula acustica of the sacculus. The edge 

 of the otolith is divided by radial grooves into lobes which are chiefly de- 

 veloped on the concave surface, and the central part of the concave surface 

 projects slightly as a convexity. Fig. 16, pi. viii., shows the appearance 

 by transmitted light of a transverse slice as above described. There is a 

 central opaque nucleus surrounded by successive laminae which are thicker 

 in the parts corresponding to the edges of the otolith than in those 

 corresponding to the surfaces. The nucleus is nearer to the convex 

 surface than to the concave. The nucleus is surrounded by a number of 

 opaque laminae, and these are succeeded by a number of more transparent 

 ones. Then comes another zone of opaque laminae, while the most external 

 are again more transparent. According to my interpretation, the opaque 

 zone represents the deposit of one summer, the transparent that of one 

 winter, so that the two zones together represent the result of one year's 

 growth and indicate one year of age. The fish from which the otolith 

 figured was taken was therefore two years old. 



For practical purposes, to determine the age of a number of specimens 

 quickly, I find the best method is to examine a few scales in water, 

 noting the number of winter zones, and the age apparently indicated, 

 and then to extract an otolith by splitting the skull in the median 

 plane, and to cut a transverse slice of the otolith in the manner described 

 above. In this way the conclusions drawn from the scales can be tested 

 and confirmed or modified. It may be asked why I have not prepared 

 thinner and more perfect transverse sections of the otolith by grinding 

 down thick slices. I have tried this method in the plaice, and not found 

 it very successful. The piece to be ground down, after one surface has 

 been ground smooth, must be fixed on a glass slide with Canada balsam in 



