AS AN INDEX OF AGE. 3 



In order to explain the formation of superimposed scales, Leuwenhoeck 

 believed tliat the growth of scales, as that of hair, feathers, horns, and trees, 

 ceases for a very short period at the end of the first year, and that scales are 

 afterwards formed beneath and add themselves to the lirst. As the scales of a 

 two-year-old fish exceed those of a one-year-old fish in size and dimensions, it 

 follows that the scale of a two-year-old fish is partly covered by the first-year 

 scale and extends beyond it. The same takes place for the following years, 

 and thus the scales of a ten-year-old fish are composed of ten scales, secondarily 

 superimposed the one upon the other, and fused so intimately with one 

 another that they cannot be easily separated without tearing the scale into 

 pieces. If, in the scales of fishes, new scales did not weld themselves each 

 year to the old scales, tlien the scales of very large fish would of necessity be 

 very thin and fragile. Leuwenhoeck restated these views in a later paper.* 



Iieaumur made a study of the silvery substance of scales, sometimes known 

 as "Tessence d'Orient."! He stated that this substance consists of a 

 mass of an infinite number of small and very irregular bodies. These bodies 

 are extremely thin, but of great solidity. He found this substance present 

 almost solely on the internal surface, not on the external surface, of the scale. 

 He held that this substance is covered over by membrane, and contained 

 in vessels or tubes which extend in a direction transversely perpendicular 

 to the length of the scale. Ec'aumur affirmed that this silvery matter con- 

 tributed directly to the growth of scales. He agreed with Leuwenhoeck that 

 each scale is composed of an indefinite number of layers, of which the largest 

 are those nearest the body of the fish. Speaking of the concentric lines, 

 Reaumur Avrote "that they occupy the border of each layer, of which they 

 mark the limit, and that they indicate the different degrees of growth in 

 scales, just as the analogous markings indicate the growth of shells." 



As to the radiating grooves in scales, Reaumur believed that they lodged 

 blood-vessels. He also gave a good description of the scales of the lateral 

 line, pointing out that scales from that region have a small canal on their 

 external surface. These small canals abut end to end, thus forming a continu- 

 ous channel, which apparently serves to carry away the mucus formed on the 

 bodies of various fishes. 



Roberg reproduced a figure of the scale of the eel, previously given by 

 Leuwenhoeck. I 



In his Memoir on the Carp, Petit dealt with the following points in connec- 

 tion with their scales, but only in a very brief manner : the mode in which 

 scales overlap each other, the dimensions of scales in different regions of the 

 body, the mode in which scales are enclosed in the skin, the furrows (sillo)is) 

 on the upper surface of the scale, the silvery matter on the lower surface, 

 etc. He, however, gave a mucli more detailed description of the scales of the 

 lateral line. § 



A few observations on scales are found in the writings of Schaeffer. He 



also gives figures of scales from five difi'erent species of perch. He notes the 



external characters of scales, and their variation in different parts of the body, 



but does not go into their detailed structure nor development. || 



* Leuwenhoeck, 1716. t Reaumur, 1716 and 1718. X Roberg, 1717. 



^ Petit, 1733. ll.^Schaeffer, 1761. 



y 



