14 THE PERIODIC GROWTH OF SCALES IN GADIDiE 



sometimes situated freely side by side, or the one above the other, sometimes 

 they increase directly to form the asperities and teeth on the posterior border 

 of scales {Perca jluviatili$, Acerina cernua), sometimes they fuse together at 

 their margins, forming a united mass, a layer of the scale. Leydig asks of 

 what nature these corpuscles are which vn fusing come to produce scales. In 

 considering the role of these corpuscles in the production of scales, he con- 

 siders them as analogous to the free globules of Czermak, wliich on fusing 

 together produce dentary substance. 



According to him, the grooves on the scales of the lateral line show 

 a different texture from the rest of the scales. They are rather true osseous 

 products superadded to tlie scales. In another j^aper Leydig gives observa- 

 tions on the structure of scales in Pohjptenis bichir* which does not 

 specially deserve attention in a })aper dealing mainly with cycloid scales. In 

 a later work Leydig deals with the sulyect of the corpuscles in scales of 

 various genera;! but this consists in the main of a reproduction of his 

 previous work in the first paper mentioned. 



Hollard issued a monograph on the family Balistidff.t The disposition of 

 tubercles and spines on the scales, their grouping and mode of formation, 

 have chiefly engagetl his attention so far as he takes up the subject of scales in 

 this monograph, Hollard § published a second monograph on the Ostracions, 

 in which he gave a detailed description of the tegumentary and scale systems 

 in this family. He held that the spines on scales are of value for purposes of 

 specific classification. 



Steenstrup issued a paper in Avhich there is an interesting note in direct 

 connection with the main subject of my paper. || He says, "The scales of 

 osseus fishes, Cycloid, Ctenoid, and Ganoid, persist during the entire life of the 

 fish. They grow with the growth of the animal. The scaly covering of lish 

 is consequently composed of the same number of scales during the entire life 

 of the animal. This is so true, that allied species may be distinguished with 

 certainty by the number of scales in each longitudinal line." Steenstrup 

 states that the case is very different in cartilaginous fishes, that placoid scales 

 do not grow with the fish. Their size never exceeds certain limits, and their 

 existence is only temporary. They fall off continually and give place to others. 



Owen gives a brief note regarding the scales of the tunny, and a description 

 accompanied by a figure of the scales of the eel. H 



Blanchard published a work on the fresh-water fishes of France.** Li this 

 work he does not give any detailed description of the internal structure of 

 scales, but he gives figures and descriptions of their external appearance for 

 a number of species. He finds it difticult to agree with Agassiz's idea of the 

 mode of scale growth by the successive addition of new plates or laminae to the 

 inferior face of previously existing ones, and in regarding the concentric lines 

 as the edges of those plates, as, according to Blanchard, the number of 

 concentric striaj is as great in very small as in very large fish of the same 

 species. Blanchard brought forward a novel idea as to the function of scales, 

 namely, that they fulfil a role in the respiratory function, varying in degree in 



* Leydig, 1854. f Leydig, 1866. t Hollard, 1853, 1854. 



§ Hollard, 1857. |1 Steenstrup, 1861. 1[ Owen, 1866. 



Blanchard, 1866. 



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