672 
PROFESSOR WILLIAMSON ON THE STRUCTURE AND 
original structure. It has wholly lost its cartilaginous aspect, but its inner surface 
merges in the true cartilage and is inseparable from it. 
There can be no doubt that each of these osseous plates is formed on a plan 
which is identical with that already referred to in mammalian bones, as explained 
by Professor Sharpey. The same processes are exhibited in both instances and fol- 
low the same order : the altered arrangement of the cartilage-cells, the calcifica- 
tion of the intercellular substance, and the production of small cavities in the bone 
corresponding with the position of, and owing their existence to, the cartilage-cells, 
are phenomena common to the growing bone of the mammal and the plates covering 
the cartilages of the Ray. But whilst in the former case this type is only transitional, 
leading to the formation of a still higher and more complex series of structures, by 
which its place is permanently occupied, in the latter instance it is the permanent 
state. Its physiological condition remains the same in the matured individual as in 
the foetus, being only affected by the increase of bulk ; and it furnishes another ex- 
ample of that permanent arrest of an early process in the development of the higher 
animals, of which so many instances are now on record. 
On turning from the surface to the interior of the vertebral column of the Thorn- 
back Ray, we find that a modified form of the same physiological condition exists. 
It is well known that in the interior of the soft vertebral column of many of the car- 
tilaginous fish, there exists a chain of calcareous elements, representing a series of 
osseous vertebral centres, but which have not advanced to a complete state of deve- 
lopment. In the Ray, each of these exhibits the two concave extremities, connected 
together by divergent plates*, whilst a continuous canal is left open through their 
centres, within which the chorda dorsalis is lodged. In fig. 28, the vertical section has 
divided this osseous centrum, midway between its two concave extremities, exposing 
the canal of the chorda (28 e), the calcareous ring by which it is surrounded (28f), 
and the fine radiating laminse which connect the two extremities together. Of the 
latter, one proceeds upwards (28 g) to form the floor of the neural canal, two pass 
upwards and outwards (28 h) towards the parapophyses, and the two lower ones, 
which are bifid externally, are directed downwards and outwards (28 i). 
On examining these several elements of each centrum under a still higher magni- 
fying power, we see that they have precisely the same internal structure as the ex- 
ternal osseous plates, modified only by minor differences in the forms and distribu- 
tion of the small cavities, dependent upon the relative portions of the centrum which 
they occupy. In that which immediately surrounds the canal of the chorda (28 /), 
they exist as flattened cells, arranged in concentric rows ; but in the projecting 
plates, 28g,h,i, they are spherical and are disposed in radiating lines directed from 
the centre towards the periphery. 
On examining the condition of the cartilage near the line of junction with the bone, 
we find very similar appearances to those seen in fig. 31, only the direction of the rows 
* See Mr. Millar’s Footprints of the Creator, fig. 8 b, p. 43. 
