TEGUMENTARY ORGANS 



381 



■extending as far as the most superficial vascular canals, and a deep 

 portion beneath these belonging to the enderon. 



However, all these points can only be decided by a much more 

 extensive series of investigations, principally directed to the ascertain- 

 ment of the position of the protomorphic line and of the direction of 

 growth of the constituents of every scale, than I have hitherto had 

 time or opportunity to carry out ; and as the attention of other 

 ■observers does not appear to have been directed to these particular 

 points, the question must for the present remain undecided. 



Professor Williamson in his valuable and philosophical contributions 

 to our knowledge of this subject (Phil. Trans. 1849-1852) laid the 

 foundation for a comprehension of the mode of development of fish- 

 scales, by pointing out that Agassiz's views, though essentially true, 

 yet require a certain modification. 

 For though a fish scale does really 

 grow by the apposition of layers to 

 its deep surface, as Agassiz asserted, 

 yet it is not included in a sac of the 

 epidermis (if by that term we are to 

 understand the ordinary cellular ec- 

 ■deron) ; and it is also true that its 

 deeper portions grow by their super- 

 ficial surface. Professor Williamson 

 points out, in fact, that every fish- 

 scale consists of at least two portions, 

 a superficial homogenous, or at most 



canaliculated, laminated layer, the ganoin (so-called enamel or horny 

 layer of authors), and a deeper, also laminated, frequently fibrous or 

 ■osseous portion commonly traversed by Haversian canals. Now 

 these two portions have a certain independence in their mode of 

 growth, at any rate after their first formation, as may be easily 

 understood by the accompanying diagram {fig. 307.), which represents 

 a series of imaginary sections of scales from their first growth 

 onwards ; a, is the protomorphic plane ; b, b" , the deep ecderon ; b' , 

 the superficial cellular ecderon, and the line x, the centre of the scales 

 from which development commenced. 



Suppose A to be the youngest scale, constituted merely by a 

 thickening and calcification of the deep ecderon, which in B has 

 added several layers by apposition to its inner surface, all of which 

 retain the ganoin structure except the deepest, which becomes 

 fibrous in its texture, and forms the commencement of the "Lepidine" 

 layers of the scale ; — these layers, however, being as much a part of 



Fig. 307. 



