686 
PROFESSOR WILLIAMSON ON THE STRUCTURE AND 
into which is fitted the thin osseous vertical plate, which contributes to the forma- 
tion of the interorbital septum. Though this bone is everywhere in close connection 
with the cartilage and bears a definite relationship to it, the two are really separated 
by an appreciable intervening cavity, in which the new membraniform bone is chiefly 
produced. No chondriform bone is visible in the cartilage. The anterior extremity 
of the presphenoid in an advanced stage of its growth exhibits some similar ap- 
pearances. I do not believe that the above has been the primary foetal condition of 
these bones. It is probable that both the presphenoid and the vomer are but exo- 
genous prolongations from the sphenoid, and that the primary centre of their osseous 
development has been in the interior of the latter bone, which has been developed 
in conformity with the plan followed in other parts of the skeleton. The presphe- 
noid and vomer would thus be the result of an unsym metrical development of the 
lower half of the osseous structure produced around the primary cartilaginous matrix. 
We shall obtain some additional light on this point from an examination of the same 
bone in some other species of fish. 
In the palatine bone a thin film of cartilage extends through its interior, and 
appears externally between the two condyles which respectively articulate with the 
maxillary and prefrontal bones, where it is of considerable thickness. Through- 
out a great extent of the thin posterior squamous portion of the bone the cartilage 
does not exist, the superior and inferior series of membraniform lamellae being in 
direct contact. 
None of the topics suggested by ichthyotomy have been productive of more elabo- 
rate disquisitions and debates than the opercular bones. Whilst some authors have 
regarded them as constituting a part of the endoskeleton, others have referred them 
to the exoskeleton, and considered them as being enlarged and modified scales. The 
latter opinion, which is the one entertained by M. Agassiz, has been recently com- 
bated by Prof. Owen, who, from an examination of the operculars of the Carp and 
Goldfish, concluded that their development ‘‘ is effected in precisely the same way as 
that of the parietal and frontal bones. The cells which regulate the intus-susception 
and deposition of the earthy particles, make their appearance in the primitive blastema, 
in successive concentric layers, according to the same law which presides over the 
concentric arrangements of the radiated cells around the medullary canals in the 
bones of the higher vertebrata*.” The above description, based as it is upon the 
long-prevailing idea that the lacunae of the Haversian canals amongst the vertebrata 
owed their existence directly to the cells of the cartilage, does not, of course, agree in 
its details with my own observations ; but it is perfectly accurate in its general bear- 
ing upon the moot question as to the nature of the opercular bones. The scales of 
the Pike are formed in precisely the same way as all the cycloid scales previously de- 
scribed. The structure of the opercular is wholly different, but is closely accordant 
with that of the bones of the endoskeleton. 
* Lectures on Comparative Anatomy, vol. ii. p. 139. 
