TEGUMENTARY ORGANS 38' 



body of the spine being 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, be- 

 tween its upper surface and the 

 under surface of the ecderon 

 (/). Examining it in the per- 

 fect state, then, it would appear 

 that the spine is included in 

 a sac of the enderon ; and 

 this appearance is very much 

 strengthened if dilute hydro- 

 chloric acid be added, by which p,Q ^^g 

 the enamel layer (a) is dissolved 



out, and the structureless membrane enclosing the spine rendered 

 distinct ; 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 inwards of the extremity of this truly 

 ecderonic structure. In fact, inasmuch as the base of the spine grows, 

 like its shaft by continual addition 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 (Jig. 307. c). 



Now suppose such plates as these to have acquired their maximum 

 in width and minimum 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 

 arrangement of the scales in the cycloid and ctenoid fish (_;f^. 309.).^ 



A careful study of the scales of that remarkable animal the 

 Sturgeon, which exhibits in this, as in so many other characters, its 

 intermediate position between Teleostian and Plagiostome fishes, 

 appears to me to throw still further light upon the difficulties of scale 

 development. 



The scales of the sturgeon are large, slightly convex, rhomboidal 

 plates, set obliquely in the skin, so that, while the posterior two-thirds 



^ 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 completely included in the enderon, and is wholly covered by the cellular 

 ecderon. I believe this appearance of inclusion in a complete sac to proceed 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. 



