482 



TEGUMENTARY ORGANS. 



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 Pro- 

 fessor Williamson, will take place. If the ex- 

 planation 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 de- 

 velopment. 



All that I have observed leads me to con- 

 firm 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 papilla 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 ec- 

 deron, which before went on, a deposit of cal- 

 careous matter takes place at the boundary- 

 line, and the structureless band remains as 

 structureless or " basement " membrane, in- 

 vesting the future spine. The deposit in- 

 creases until the enderonic pulp occupies but 

 a very small space, or even completely disap- 

 pears, 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 (fig. 

 308.). 



Fig. 308. 



once formed, the calcareous additions which 

 give origin to its base (c) gradually cease to 

 be in exact apposition with the original 

 protomorphic zone ; and in proportion as the 

 base of the spine extends, have we a wider and 

 wider interval, occupied by the tissue of the 

 enderon, between its upper surface and the 

 under surface of the ecderon (/). Examin- 

 ing it in the perfect state, then, it would appear 

 that the spine is included in a sac of the en- 

 deron; and this appearance is very much 

 strengthened if dilute hydrochloric acid be 

 added, by which the enamel layer (a) is dis- 

 solved out, and the structureless membrane 

 enclosing the spine rendered distinct j while 

 its continuity with that structureless layer 

 which bounds the enderon is at once obvious. 

 From its development, however, it is clear 

 that this is a simple appearance, and that the 

 apparent sac results from the projection in- 

 wards of the extremity of this truly ecderonic 

 structure. In fact, inasmuch as the base of 

 the spine grows like its shaft by continual ad- 

 dition to its inner surface, while its apex is 

 unquestionably an ecderonic structure, this 

 base might be considered to be enveloped in 

 an involution of the protomorphic plane of the 

 ecderon (fig. 307. c). 



Now suppose such plates as these to have 

 acquired their maximum in width and mini- 

 mum in height ; furthermore, imagine them to 

 be so closely set in the skin that the posterior 

 edge of one over-rides the anterior edge of the 

 one next behind it, and we have the exact ar- 

 rangement of the scales in the cycloid and 

 ctenoid fish (fig. 309.).* 



Fig. 309. 



It is particularly to be remarked, however, 

 that the projecting body of the spine being 



Scale of the Roach (Leuciscus.) 



A, section ; B, surface. 



* The flexible cycloid scale of the eel presents an 

 exact parallel to the tooth-like placoid scale of the 

 skate, except that it is flat instead of conical, and 

 that, in the adult state, the scale appears to be com- 

 pletely included in the enderon, and is wholly 

 covered by the cellular ecderon. I believe this 

 appearance of inclusion in a complete sac to pro- 

 ceed simply from the smallness of the original point 

 of contact of the scale with the cellular ecderon, 

 and the rudimentary state in which the whole organ 

 remains. 



