24 THE PEKIODIC GROWTH OF SCALES IN GADID^ 



the border of the scale to the centre of growth ; a certain number of them do 

 extend over all this distance, but the others usually only run over a portion 

 of the radius. Some of the radiating grooves commence at the periphery and 

 stop almost immediately, others extend a little further, and others still further 

 without reaching the centre of growth. Ridges may also be seen commencing 

 at only a short distance from the margin of the scale, sometimes terminat- 

 ing at the centre of the scale, and at other times terminating after a short 

 passage. It is clear that there occur grooves whose course is reduced, as they 

 only extend over a minimum portion of the radius. Grooves occupying the 

 median portion of the anterior area are, as a rule, longer than those at the 

 sides or lateral areas of the scale. This also holds true for the grooves in the 

 posterior region of the scale. When the radiating grooves are wide, regular, 

 and very close together, the scale surface appears as if it had been cut into a 

 series of bands or triangular tongues, with the apices turned towards the 

 centre of growth (see anterior region of scale of sole). 



The concentric or transverse grooves are situated between the concentric 

 ridges, and are more or less parallel to them. These grooves are, as a rule, only 

 found on a limited part of the scale surface, and they occur more frequently 

 in the periphery than in the part surrounding the centre of growth. These 

 concentric grooves may be very narrow (whiting, Ophidium) or very wide 

 (lateral areas in various Pleuronectids). When very wide concentric grooves 

 co-exist with radiating grooves equally wide, the surface of the scale becomes 

 divided up into calcareous areas of varying size. These areas may be irregular 

 (posterior area of various Pleuronectids, scales of Gadus molva), or they may be 

 regularly rounded and in the form of small medallions (scales of eel). The 

 number of radiating grooves varies much in different scales from the same fish ; 

 these variations become very apparent in comparing scales from different 

 regions of the body, rudimentary with well-developed scales. In extremely 

 rudimentary scales grooves may not exist. In scales from the same region of 

 the body the number of radiating grooves does not vary to nearly the same 

 extent. The number of grooves of an individual scale is capable of varying 

 with age. As regards the transverse or concentric grooves, there does not 

 appear to be any doubt that these grooves, Avhich are situated between the 

 concentric crests, are formed at the same time as the latter. In regard to 

 the radiating grooves, it appears that they multiply during the growth of 

 the scale, at least in a very large number of cases. 



If the number of grooves in scales increases with age, it may also become 

 reduced. This fact appears true for the transverse grooves, as in scales in 

 which these grooves are found (whiting, Ophidium) one usually finds them 

 much more pronounced towards the periphery than towards the centre, where 

 they may completely disappear. As to reduction in the number of radiating 

 grooves, !Mandl observed that they disappeared in old fish belonging to the 

 genus Abramis, and in other old scales they evidently disappear in the area 

 round the centre of growth. Baudelot remarks that " up to the present time 

 (1873) the grooves on scales have not been explained in a satisfactory manner." 

 Mandl regarded them as canals serving for purposes of nutrition of the scale ; 

 Peters as suture lines which rendered possible the growth of scales. William- 

 son held that they were erosions effected at the expense of the superior layer 



