386 TEGUMENTARY ORGANS 



difference being, that in the latter case the development of the upper 

 layer is less rapid than that of the lower, while in the former they are 

 coincident ; that soft membranous separation therefore, which exists 

 between the two layers anteriorly, is far less developed posteriorly ; 

 and the soft continuation of the scale which is flat anteriorly, is in- 

 flected posteriorly ; the process of addition being otherwise the same. 

 Suppose, now, that each detached calcareous centre of ossification as 

 it is added to the posterior margin of the scale, instead of being 

 flattened, were produced into a spine as in the rays, then it is perfectly 

 clear that instead of a cycloid scale, the result would be a serrated 

 ctenoid scale. And this appears to be exactly what takes place in 

 the scales of the perch, according to Prof Williamson's description. 



From all this, I think, we arrive at Prof Williamson's conclusion, 

 that fish-scales are essentially tegumentary teeth ; that like the latter 

 organs, they result not from the calcification of the cellular ecderon 

 covering those folds of the integument, upon which they are developed 

 and which correspond with the dental pulp, but by a calcareous 

 deposit taking place beneath this, in what represents a deep layer of 

 the ecderon ; finally that it is, for the present, an open question whether 

 the deep layers of all scales are produced by a continuation of this 

 process, or whether in some cases a deep truly enderonic structure- 

 may be added to this superficial ecderonic constituent to constitute 

 the perfect scale. A process of the latter kind would, at any rate, 

 find its parallel in the eventual union of the teeth of many fishes with 

 their jaws, and in that of the plates of the chelonia with the vertebral 

 elements. 



i^ 3. Histology of the tegiimentary organs. — Having thus arrived at 

 a general idea of the mode in which the various forms of integu- 

 mentary organs are produced from the primary morphological 

 constituents of every integument, we have now to consider their 

 minute histological elements and the mode in which these proceed 

 from the indifferent tissue of which all organs are primarily com- 

 posed. 



The tegumentary tissues, like all others, are produced by the meta- 

 morphosis of the periplast of the protomorphic or indifferent tissue 

 from which they take their origin, the endoplasts, to all appearance,, 

 taking but little share in the metamorphic processes. The chemical 

 metamorphosis of the periplast may be either into horny, chitinous,, 

 calcareous, or cellulose matter ; in form it may become fibrous, 

 laminated, vacuolated, bony, prismatic, &c. 



As a general rule, the endoplasts tend to disappear, pari passu^ 

 with the metamorphosis in form and composition of the periplast ; but. 



