AS AN INDEX OF AGE. 13 



which agree in their direction with the membranous laminae previous to 

 calcification. The laminae also exhibit a number of vertical cleavages or 

 fissures. " Tlie middle layer then is produced by the formation and coal- 

 escence of the small lenticular bodies, through the agency of which the 

 calcification of the membranous laminae is effected. This calcification per- 

 meates the entire extent of the upper and earlier-formed lamellae, whilst, 

 with the exception of a few isolated granules, it has been confined to the 

 margins of those which are inferior and of more recent growth." 



The superior layer of the scale differs both in structure and in mode of 

 origin from the median and inferior layer. This superior layer is the one by 

 various modifications of which all the ridges and tubercles seen on the 

 surfaces of scales are produced. In vertical section it frequently shows an 

 undulating outline and has traces of a lamellar formation (the lamellae 

 being homogeneous and devoid of structure), the more external being parallel 

 with tlie upper surface of the section. The radiating lines (nutrient canals 

 of Mandl) are produced simply by the absence of superficial tissue along 

 their course. While these radiating lines are not nutrient canals, as was 

 supposed by Mandl, neither do they pass through the entire calcareous 

 portion of the scale and reach the underlying soft tissues, as was maintained 

 by Agassiz : they only do so at the margin of the scale, where the median 

 layer is not yet developed ; but towards the centre, where the median layer 

 exists, these grooves do not pass through it. The ridges intervening between 

 these radiating lines are of some thickness, and are transversely subdivided by 

 a large number of small ridges. These ridges are really the concentric lines 

 seen on the surfaces of most cycloid and ctenoid scales. The superior layer of 

 the scale covers the entire surface of the scale even to its extreme periphery, 

 but the median ceases to exist at some little distance from the margin. The 

 growth of the superior layer is effected at its upper surface by the calcifica- 

 tion of a thin superficial membrane which covers the scale at the same time 

 that the corresponding though different process is adding to the lower surface 

 of the median layer. He says "it thus becomes manifest that these concen- 

 tric ridges are not lines of growth, as thought by M. Mandl, but the result of 

 a peculiar arrangement of the superficial tissue of the scale, a conclusion 

 which accords with that arrived at by M. Agassiz." After a description of 

 the scales of the carp, pike, salmon, perch, he says, " The question which now 

 suggests itself is, what relation does the superior investing membrane bear to 

 the inferior fibrous portion 1 " 



To this question, however, he is unable to give anything more than 

 hypothetical answers (see page 654, Williamson) and continues, "Be the 

 process of its genesis what it may, we have here demonstrative evidence of 

 the existence of such a superficial film of soft membrane as is essential to my 

 hypothesis, accounting for the peculiar structure and growth of tlie upper- 

 most layer." He further regarded the substance of the superior layer as 

 probably identical with the ganoin existing in Lepidosteus, Lepidotus, and 

 their allies. 



Leydig gives a description of the structure of scales,-" in which his reference 

 to the corpuscles of Mandl is the most important point raised. Tiiese are 



* Leydig, 1851. 



