DEVELOPMENT OF THE SCALES AND BONES OF FISHES. 
691 
petrorsals, alisphenoids and other hones of the cranium also correspond in their mode 
of growth with the same bones in that example ; only, in accordance with what has 
just been remarked, the cartilage in the centre of each bone has been more or less 
absorbed, and only exists permanently as an irregular external ring, separating the 
peripheries of the two osseous layers ; being thick at its margin and becoming thinner 
as we approach the centre of the bone. The cancelli of the latter portion appear to 
have been formed as before. 
The dentary bone also exhibits a cylindrical rod of cartilage projecting from its pos- 
terior angle, and extending across the inner surface of the mandible. The vomer and 
presphenoid are developed upon the under surface of the interorbital and nasal car- 
tilage ; only the projecting osseous lamina, contributing to the formation of the inter- 
orbital septum (which is almost rendered complete by a thin upward expansion of 
the cartilage), is still larger than in the Pike. In fact it may be briefly stated, that, 
with one exception, the processes of growth are the same in the Pike and in the 
Perch, only modified in the latter fish by the greater extent to which the osseous 
tissue has displaced the cartilaginous. 
The exception to which I here refer is the vertebra. This bone is wholly osseous. 
No cartilaginous segments enter into its composition, and the various apophyses are 
firmly anchylosed to the bone. I have not been able to obtain any evidence indicating 
that cartilage has ever entered into its structure. It appears probable, that in the first 
instance an osseous ring has been developed in the membrane surrounding the chorda 
dorsalis, and that the sole way in which its subsequent growth has been effected, has 
been by peripheral additions of the same kind of bone, variously arranged. 
The same peeuliarities exist in the vertebrae of the Common Plaice {Platessa 
vulgaris, Flem.). I find no trace of cartilage in connection with them, even in the 
youngest examples which I have been able to obtain. Numerous irregular longitu- 
dinally disposed plates radiate from the centre towards the periphery, and are con- 
nected at right angles by other smaller laminae which bind them together. There is 
an attempt at the formation of four distinct segments like the osseous ones in fig. 41, 
and it is just possible that in a very early foetal condition, cartilage may have existed 
in the intermediate spaces. The way in which the bones of the Pike appear perma- 
nently to typify the earlier stages of the osteo-genesis of many other fish, would lead 
us to suspect that such may have been the case. If so, the cartilage has wholly disap- 
peared very early; since we soon find that these intervals are also traversed, like the 
rest of the vertebra, by a few transverse laminae, which thus convert them into large 
cancelli occupied by fat-cells. In the Plaice, the parapophyses are certainly nothing 
more than prolongations of the radiating plates, and not formed from independent 
centres of ossification : I have some doubts whether the neurapophyses have not been 
produeed in the same exogenous way. 
In the vertebrae of the young Haddock {Morrhua oeglejvnus, Cuv.), we find the same 
peculiarities as in those of the Perch and Plaice, but the outlines of the deep lateral 
