6 THE TERIODIC GliOWTIl OF SCALES IN GADID^E 



Reaumur. Along with his description there is an analysis of this substance 

 by Rose.* 



Agassiz expressed himself in the following manner in regard to the structure 

 and development of scales : — f 



"Scales are contained in mucous cavities or in small sacs formed by the 

 ' chorion,' to which, however, they do not adhere by vessels. They are 

 formed of lamellce, of hornj' or calcareous layers, superimposed the one on the 

 other, which are secreted at the surface of the chorion ; these layers attach 

 themselves successively to the inferior surface of the preceding, to which they 

 weld themselves by layers of hardened mucus. In order to obtain a true idea 

 of this development, it is necessary firstly to observe it in those genera of fish 

 in which the scales appear to show these arrangements in the simplest state, 

 for example, in the Eels, the Blennies, Cobitis, and Leuciscus. It is easy to 

 assure oneself that the concentric lines of the anterior border and those of the 

 posterior border are continuous the one with the other." 



In order to support his theory, which after all is none other than that 

 of Leuwenhoeck, Agassiz appealed to the following : — 



"After having macerated scales for some time in water, one can easily," he 

 says, " divide them up in a large number of layers or plates of greater or less 

 thickness, and of difiereut size, but all of which have the form of the scale. 

 These plates are superimposed in such a manner that the smallest occupy the 

 centre of the scale, and form its interior part, while the largest, bordering the 

 preceding, are successively welded to their inferior face. Thus one sees that 

 the concentric lines which are visible on the exterior surface of scales are 

 simply the borders of plates which compose them." 



The radiating grooves (sUlofis rayonnani^) Agassiz regarded as channels at 

 the margin of the external surface, which connect one layer with another, and 

 multiply during the growth of the scale. In this work Agassiz introduced 

 his well-known classification of fish into four orders according to the nature of 

 their scales, the Placoid, Ganoid, Ctenoid, and Cycloid orders. 



Mandl, J well known as the opponent of Agassiz, held widely different views 

 in regard to scale structure and development from those of all the authors 

 previously mentioned. 



He attempted to establish the existence of an internal life and true 

 organisation within the scale. According to him, most scales are composed of 

 two superimposed layers, superior and inferior. The superior layer shows the 

 structure of cartilage with corpuscles, the inferior layer consists of lamellae 

 which recall the structure of fibrous cartilage. 



In the superior layer he describes longitudinal canals, cellular lines, and 

 corpuscles. 



Loivjitudinal Canals. Under the term ^Hoyigitudinal canals" he describes 

 the grooves which radiate out from the centre to the periphery of the scale. 

 According to Mandl, these lines show all stages of formation, from that of a 

 simple groove to that of a perfectly formed canal. These canals lead to a 

 common point, the focus, which is a centre of nutrition, a point where tissue 



• Ehrenberg, 1833. t Agassiz, 1834. t Mandl, 1839. 



