690 
PROFESSOR WILLIAMSON ON THE STRUCTURE AND 
chorda, are apparently analogous to the central nucleus, seen in the centres of the 
carpal and stylo-hyal bones (37 c), having a similar structure and origin. In each 
case its continued development is independent of the immediate proximity of any 
progressing growth of membraniforrn bone. The chondriform bone fringing the 
growing cancelli of the apophyses, on the other hand, bears more relation to that 
seen in the enlarging condyles of the epitympanic (38 c) and in the centres of the 
parietals and aiisphenoids. When the cartilage at the base of each apophysis is 
absorbed, to make way for new cancelli, the contained granules of chondriform bone 
must necessarily disappear along with it. 
We thus see, that, notwithstanding the great difference existing between the com- 
plex form of a Pike’s vertebra and its more simply constructed carpal bone, the two 
are developed according to a common plan. Whilst the distinction between chondri- 
form and membraniforrn bone is maintained in both cases, the two tissues invariably 
bear the same relation to each other. 
Before leaving this portion of the subject, the close resemblance between the 
general direction of the divergent segments of membraniform bone in the vertebra of 
the Pike, fig. 41, and those of chondriform bone in the corresponding portion of the 
Thornback Ray, fig. 28, deserves a passing notice. The chief difference a rises from the 
circumstance, that, whilst in the latter the two inferior plates (28 i) are separated by 
an intervening prolongation of cartilage, in the former (41 d) they have been united 
from the first. The substitution of the one kind of bone for the other in the two 
cases is an interesting fact. 
After having thus acquainted myself with the structure and growth of the bones 
of the Pike, I pursued the same plan in reference to the bones of a number of other 
osseous fish, into the permanent composition of whose skeletons a much smaller 
amount of cartilage enters. 
In the skeleton of the Perch {Perea JiuviatiUs, Linn.), I found that nearly all the 
bones, excepting the vertebrae, exhibited similar conditions to those existing in the 
Pike, modified only by the greater extent to which the cancelli have encroached upon 
the respective cartilages. The epitympanic bone taken from each of these fish pre- 
sents a good illustration of the difference between them ; on viewing their external 
aspect, the two bones appear to be very similar; but on making vertical sections of 
that of the Perch, we find that instead of the terminal cartilage being prolonged 
through the interior of each condyle, and almost reaching the centre of the bone, as 
in the case of the Pike, it only exhibits at its proximal part a very obtuse conical ter- 
mination, penetrating but a little way into the concave extremity of the osseous por- 
tion of the condyle; its ossifying surface however still presents the peculiar film of 
chondriform bone. The remainder of the interior of the condyle consists of numerous 
membraniform cancelli, formed by successive encroachments upon the cartilage ; 
whilst the external cylinder, also membraniform, has been produced in precisely the 
same way as the corresponding portion of the epitympanic of the Pike, fig. 38 d. The 
