LIFE HISTORY OF LAKE HERRING OF LAKE HURON 331 



While apparently, then, the outer zones of the scales expand with age, perhaps 

 the inner zones undergo a contraction at the same time. The data of total lengths 

 of Table 22 indicate that apparently they do contract. In the 1918 year class the 

 diameters of the scales of the 5-year fish averaged consistently shorter than the 

 corresponding diameters of the scales of the 4-year fish (compare values 4.86 and 

 4.58 of the third growth year). The scale diameters of the 6-year herring, however, 

 averaged longer than those of the 4-year fish in the first growth year, the same in the 

 fourth year, but shorter in the second and third growth years. In the 1919 year 

 class the scale diameters of the fourth and fifth age groups, which, except for the 

 fourth growth year, averaged approximately the same, were consistently shorter 

 than those of corresponding years of the third age group. So, also, the scale diam- 

 eters of the 6-year fish of this year class averaged consistently shorter than those of 

 corresponding gi-owth years (the fifth excepted) of the 3, 4, and 5 year fish. The 

 1920 and 1921 year classes show even more strildng results. In these fish, without 

 any exception, the scale diameter of a certain growth year decreases as older age 

 groups are employed. 



It is to be noted that the differences between the total lengths of the scale diam- 

 eters of any two age groups of a year class diminish after the second or third year of 

 life (compare, for example, the diameters of 'the 1920 year class). By referring to 

 the increments of Table 22 it may be seen that as a result of this decrease in the differ- 

 ences the order of scale increments is reversed after the second or third growth year. 

 Whereas, the scale increments of the first and second growth years usually decrease 

 as the age of the fish whose scales were measured increases, those of the third year 

 change little with age, and those of later growth years increase as the age of the fish 

 increases (see, for example, the increments of the 1920 year class). 



Do the scales of herring, therefore, contract with age? Our data suggest that 

 apparently a concentration with age takes place in the first two innermost zones of 

 scales. The data likewise indicate that the third zone changes httle with age while 

 the outer zones expand. Do contraction and expansion then occur synchronously 

 in the scales of lake herring? This does not seem possible in view of what is known 

 concerning the structure of teleost scales in general. "In minute structure each 

 scale consists of an outer layer of bone, which, like the bone of the endoskeleton, may 

 either be homogeneous, except for a feeble lamination, or it may contain bone cells 

 arranged in successive layers parallel to the surface of the scale. In addition, there 

 is an inner fibrous stratum in which the fibrous bundles in any one plane cross those 

 in planes above or below them." (Bridge, T. W., in Cambridge Natural History, 

 1910, Vol. VII, p. 189.) The fibrous bundles of any one plane form a thin lamella. 

 During the growth of the scale these lamelliB are deposited on the lower surface of 

 the scale, each new lamella growing larger than and extending beyond the one most 

 recently formed. Lea (1919) found that in the marine herring (Clupea) the breadth 

 of the zones of these lameUffi "exhibits an irregular progression, broader belts sud- 

 denly appearing after a series of narrow zones, * * * that the transition from 

 narrow to broader zones takes place just where the surface of the scale shows a winter 

 ring. Thus, the elementary plates are seen to form their own system of annual rings, 

 corresponding to that of the surface layer, but otherwise differing greatly from this, 

 and more resembling that found in the scales of many salmonoids and gadoids, etc.. 



