DEVELOPMENT OF THE SCALES AND BONES OF FISHES. 
693 
ichthyotomists. Still I arn convinced that an unbiassed examination of sections 
carefully prepared from fishes of different ages, will fully corroborate the general 
accuracy of the preceding descriptions. 
The other bones of the Cod, Haddock and Plaice agree in all material points with 
those of the Perch. The radius, ulna and femur of the Haddock are ail produced by 
the development of membraniform bone on the two surfaces of a thin cartilage; but 
their thin osseous lamellae have projected far beyond the cartilaginous matrix, as in 
the case of the opercular bones, thus producing the thin squamous expansions which 
constitute so large a portion of their substance. 
Whatever may be the variations either in the form of the bones or the position oc- 
cupied by the cartilaginous matrix, some portion of the latter always makes its ap- 
pearance at the surface of the former up to a comparatively advanced stage of growth. 
As far as my present observations have enabled me to judge, this appears to be an 
invariable rule. In some bones, such as the coracoids and premaxillaries, scarcely 
any trace of the original cartilage can be found. Still even in these, though they 
consist almost wholly of membraniform bone, a small aperture may usually be de- 
tected at some point, where either a small portion of cartilage is retained, or which con- 
ducts to cancelli filled with fat-cells, by which it has been permanently supplanted. 
The structure of the presphenoid in the very young Haddock illustrates my re- 
marks respecting the same bone in the Pike. A slender cylinder of cartilage runs along 
its interior and is surrounded by concentric larnellse of membraniform bone. This 
investing osseous cylinder can be distinctly traced backward far beyond the posterior 
apex of the cartilage, even when the space left by the absorption of the latter tissue has 
become occupied by well-marked, secondary cancelli. As we proceed anteriorly, we 
find that the continuity of the cylinder becomes interrupted by a small fissure dividing 
its upper walls, through which an expansion of the internal cartilage escapes, form- 
ing the interorbital septum : this fissure gradually becomes wider as the contained 
cartilage increases in size, forming the expansion of the same tissue which rests upon 
th& vomer; the anterior portion of the presphenoid being at length reduced to an 
unsymmetrical prolongation of the lower laminae of the primary cylinder ; a trace 
of its original cylindrical character being preserved in the shallow groove marking 
its upper surface, on which the superincumbent cartilage still rests. In this case I 
have little doubt that the primary cartilaginous matrix occupied the centre of the 
sphenoid bone, of which the presphenoid appears to be only a vegetative, exo- 
genous prolongation. 
None of the bones hitherto described contain anything homologous with the cana- 
liculated lacunae seen in human bone. In those of the Eel {Anguilla acutirostris, 
Yarrell), such lacunae, of the peculiar quadrate type so prevalent amongst fishes, 
are abundant ; consequently I turned to their examination with a considerable degree 
of interest, being anxious to see whether the presence of these structures in any 
degree altered the genesis of the bones. As far as I have been able to comprehend 
MDCCCLI. 4 u 
