382 TEGUMENTARY ORGANS 



the ecderon as the former. In C the scale widening, the edges of its 

 " Lepidine " layer do not remain in contact with the ganoin layer ; 

 but it will be obvious that the re-entering angle thus formed by the 

 protomorphic line between the two, is only, as it were, a fold of the 

 deep, surface. If the two layers go on increasing in this way, 

 however, the ultimate effect will be that, although growing in reality 

 by its deep surface as before, the " Lepidine " layer of the scale will 

 appear to grow by its superficial surface, and that addition of layers 

 to the upper surface of the scale observed by Professor Williamson, 

 will take place. If the explanation here proposed, however, be 

 correct, this will form no objection to, but a confirmation of, Agassiz" 

 views. 



It will be well, however, with this clue to turn from the theory to 

 the facts of scale development. 



All that I have observed leads me to confirm Professor Williamson's 

 conclusion, that there is no real line of demarcation to be drawn 

 between placoid, ganoid, ctenoid, and cycloid scales ; all these forms 

 passing into one another. Indeed, I conceive that the only method 

 thoroughly to comprehend the cycloid and ctenoid scales is to 

 examine, in the first place, the so-called placoid and ganoid forms: 



Hermann Mayer and Leydig have shown (and the fact is readily 

 verifiable) that the scales and spines of the Plagiostome fishes are 

 formed by the gradual deposit of calcareous matter in processes of 

 the integument, which are at first coated by the ordinary cellular 

 ecderon. These diverticula, in fact, originally resemble other papillae 

 of the skin, and like them, are bounded by a structureless proto- 

 morphic layer, marking the boundary between the cellular ecderon 

 and the enderon. 



When the formation of the placoid scale commences, however,, 

 instead of the successive division and multiplication of the endoplasts 

 and the cellulation of the periplast of the ecderon, which before went on,, 

 a deposit of calcareous matter takes place at the boundary-line, and the 

 structureless band remains as structureless or " basement " membrane, 

 investing the future spine. The deposit increases until the enderonic 

 pulp occupies but a very small space, or even completely disappears,, 

 and the spine projects as a cylindrical or conical tubercle. When it 

 has attained its full length, the deposit does not cease ; new calcareous 

 matter is continually added to its inner extremity, but rather in the 

 direction of breadth than of length, so that, eventually, an irregular 

 broad plate is formed with the spine projecting from its outer surface 

 (A- 308.). . , 



It is particularly to be remarked, however, that the projectmg 



