AS AN INDEX OF AGE. 9 



maceration, raises some of the lamellre, the corpuscles disappear. Agassiz 

 thought that the corpuscles beneath are lamellre incompletely formed, and 

 those above are lamellcTe which have been broken down through the friction of 

 scales against one another. As to the fibrous layer, Agassiz believed that this 

 layer, which Mandl described as serving for the foundation of the cellular 

 substance of scales, has as little existence as the cells themselves ; in other 

 words, that its supposed existence was founded on an error of observation, 

 as all fibrous tissues (tendons, cellular tissue, etc.) produce gluten on boiling, 

 yet well-cleaned scales never produce that substance. Agassiz maintained 

 that scales do not show two distinct layers, but that the superior and inferior 

 layers have the same composition. He thought that the fibres described by 

 Mandl were due to a tearing of the younger and less consistent inferior 

 lamellae, which gave rise to the appearance of fibres ; but which was none the 

 less an optical illusion. According to Agassiz, the focus is simply the oldest 

 part of the scale, in which the superior lamellre have been worn away by 

 friction or exfoliation. Altogether, Agassiz maintained that the material 

 which IMandl had brought forward as to the detailed structure of scales was 

 (juite erroneous. Agassiz's idea as to the mode of scale formation may be 

 summarised as follows : — 



" The scales of fishes are epidermic secretions, analogous to that of nails. 

 As in nails, the scales are composed of exceedingly thin lamellae of a horny 

 nature, superimposed the one on the other in the order of their formation. 

 The secreting organ is the epidermic pouch, in which the scales are ensconced 

 at their anterior borders. The newdy formed lamellae are very soft, but of 

 the same composition as the oldest lamellae. The pouch increases in such a 

 manner that the newly formed lamellce are always larger than the older. The 

 concentric lines are reflexed parts of the borders of su]:)erimposed lamellae, 

 and these lines are more numerous in old than in young fish. Scales disinte- 

 grate or waste chiefly round the focus by friction of the scales among them- 

 selves or by exfoliation. The focus and corpuscles on the external surface are 

 simply results of this wearing down ; one does not find them in non-imbri- 

 cated scales, as in those of the eel, for example. By means of sections one 

 sees that scales are composed of lamellae, and that there are marks which 

 correspond to concentric lines. The so-called teeth or notches are simply 

 indentations of the posterior border of the lamella." 



In the following year, Peters* gave a critical review and summary of the 

 observations of Mandl and Agassiz. This author firstly gave some general 

 considerations on the structure of the skin of fishes. In a fresh-water fish, 

 one finds the following layers in the skin covering the scales : — 



1. An epidermis composed of squamous cells (the latter being very 

 abundant in the mucus of fishes). 



2. A layer of pigmented cells. 



3. The skin proper, a layer composed of fibrous connective tissue containing 

 fatty globules. 



4. An exceedingly thin membrane immediately on the external surface of 

 the scale, but distinct from the skin. On this membrane are seen concentric 



* Peters, 1841. 



