SCALE OBSERVATIONS OF THE SQUETEAGUE AND PIGFISH. 293 



and nob as necessary organs of the scale. He finds the zones of apparently unequally 

 distant circuli (annuli) which constitute the basis of the modern system of age determi- 

 nation. Variations in the number of ridges are not usually great on scales from similar 

 positions on the bodies of fish of the same species. In fish of the same species but of 

 different ages the number of ridges increases proportionately with age and consequently 

 also with the diameter of the scale. 



Klaatsch (1894) noticed that the concentric arrangement of the circuli is unusual 

 for superficial reliefs. He says that in trout the cells arrange themselves so as to corre- 

 spond exactly with the circuli. He further states: 



One might expect that the superficial scleroblast layer would cover the deeper cell layer with its 

 product so that tlie constituent part of the ridges would be taken up in the interior substance of the 

 scale. This does not, however, occur. The cells arrange themselves as they poss through the changes 

 described, so that they come to lie in the external sitrface of the ridges and contribute to their enlarge- 

 ment. They elaborate, as it were, the upper relief surface of the scale, for which the deeper cells had 

 only supplied the foundation. 



Ussow (1897) noticed the same thing, but at a later stage when the ridges had entirely 

 formed. At the stage when the reliefs occurred for the first time no such aggregations 

 existJed. It is possible that these cells later take part in the formation of the reliefs, 

 but he believes that the commencement of their formation arises at the expense of the 

 peripheral elements of the papilla. 



Tims (1906), in his work on cod scales, arrives at very different conclusions. He 

 describes the circuli as a series of scalelets with their peripheral borders turned up by 

 the pull of the stretched pocket. He notes the lateral fusion of two or more scalelets 

 which, if carried out completely, would result in the typical clupeoid scale which is 

 composed of eccentric imbricated rings. 



In the recent work of Miss Esdaile (1912) are found detailed statistics relating to 

 the circuli (which she calls annuli), their number and disposition, and, especially, 

 enumerations of their occurrence on scales of different parts of the body. She finds that 

 there is a uniform variation in their occurrence, an observation of much importance 

 in age determination. Her conclusions are : 



1. A great variation in the number of annuli and in the lengths of the scales taken from 

 different parts of the body of the same fish is clearly indicated. This was found on each of three fish 

 [Salmo salar], but the results obtained seem to be in no way correlated. 



2. It is to be noticed that in the three fish examined the number of annuli in each peronidium 

 increases from the head to the adipose fin, and then diminishes toward the tail. A similar increase 

 and decrease is found on both the dorsal and ventral sides of the lateral line. 



3. In a comparison of scales taken from positions at corresponding distances from the head on both 

 dorsal and ventral sides of tlae lateral line it is seen that, as a general rule, the scales on the dorsal side 

 have fewer aimuli in each peronidium." 



Mastemian (1913a) regards the circuli as stiffening or supporting tissue of the 

 scale. His discussion is, however, directed not so much toward the morphological 

 significance of these structures as toward their bearing on age determination. Conse- 

 quently his discussion of this subject is treated in this paper under the heading "Age 

 determination." 



o Miss Esdaile h.as adopted the word "peronidiiim'' as meaning that part of the scale which represents tlie growth ol a 

 stuniner and a winter together. 



