DEVELOPMENT OF THE SCALES AND BONES OF FISHES. 
675 
surfaces. At the inferior border of the centrum, or what would constitute the roof of 
the haemal canal of a typical vertebra, there is a similar enlarged plate of bone, also 
invested by cartilage on every side. 
On preparing a similar section from a vertebra of an older fish, we find that the 
same general arrangements of the parts exist, only some of them are altered in their 
respective dimensions. The osseous ring has encroached upon the cartilages, both at 
its inner and outer surfaces. In the additions made to the former portion, the cavities 
in the bone are compressed and arranged concentrically. In the external additions 
they are spherical, and arranged in radii, corresponding with those of the uncalcified 
cartilage. The two large plates occupying the upper and lower surfaces of the centrum 
have also increased in thickness, by additions made to each of their individual sur- 
faces. Their enlargement has been accompanied by a corresponding expansion of 
the portions of cartilage which are external to them, thus providing for their further 
development. 
On making a vertical section of one of these vertebrae in the longitudinal direction, 
or at right angles to the last, we find that the cartilage-cells external to the osseous 
ring still display the same radiating disposition, the lines diverging as they approach 
the periphery. The arrangement of the small cavities in the cylindrical bone is also 
analogous. Those which form the surface of each concave articulating extremity are 
very large, and are arranged in rows which are nearly parallel with that surface, having 
been developed around the cartilage-cells with which the latter portion is lined. Those 
which, on the other hand, constitute the exterior of the constricted osseous cylinder, 
are considerably smaller, and towards its terminal margins become subcompressed. 
We now also find that the osseous plate, constituting the floor of the neural canal, 
extends across the entire vertebra, and that its anterior and posterior extremities 
blend with the contiguous margins of the osseous cylinder. It is in fact a rudi- 
mentary vertical plate, serving the same purpose as that of the Ray, fig. 28 g; only 
instead of being developed, as in that example, direct from the osseous cylinder sur- 
rounding the chorda, it is formed at a distance from it, being attached to it only by 
its anterior and posterior extremities, the central portions of each being separated 
from one another by a semicircular mass of cartilage. As the growth of the fish 
advances still further, this cartilage continues to be encroached upon both above and 
below. I have never yet met with an example in which it was completely obliterated 
and its place wholly occupied by bone, but in very large and old fish it is nearly so, 
the plate connecting the two concave extremities being there very thick, the result of 
successive additions made to both its surfaces. The preceding remarks also apply to 
the corresponding plate on the opposite inferior surface of the centrum, only it never 
becomes enlarged to anything like the same extent as the superior one. 
It will be seen that whilst in these bones of the Dog-fish the process of ossification 
is essentially the same as in the Ray, it differs in one important particular. In the 
latter fish all the new growths are added to one surface only ; in the former, the 
