684 
PROFESSOR WILLIAMSON ON THE STRUCTURE AND 
extended into this proeess, the bone being wholly membraniform in its nature and origin. 
Internally the Haversian canals terminate in a large irregular cavity, 39 e, which 
occupies the centre of the bone. This has once been filled with cartilage, which, with 
its chondriform bone, has been removed by a process of absorption. The process is 
still going forward at the point 39 f, where the cartilage has become detached from 
the inner surface of the investing membraniform bone, leaving an intervening space 
which gradually diminishes in extent up to the point 39 g, where the cartilage and 
bone still retain their contiguity. The same process occasionally goes on in other 
parts of the structure. Thus at 39 h a small portion of the cartilage has become 
absorbed, and the interior of the cavity thus formed has been lined with a layer of 
membrane, in which calcification has produced a thin film of bone, thus forming a 
sort of cancellated cavity communicating with the Haversian canal, 39 k. 
If we examine corresponding sections of this bone from individuals of different 
ages, we shall find that the inner extremity of the cartilage recedes further from the 
centre of the bone as the fish advances in age ; the irregular cavity left by its absorp- 
tion being then more or less filled with adipose tissue. 
In the paroccipital, exoccipital, alisphenoid and parietal bones, we find that a still 
more considerable marginal development of the cartilages exists. These bones being 
in contact with others at nearly every part of their periphery instead of touching at a 
few salient condyles, their cartilages are not confined to the interiors of cylindrical pro- 
cesses, but form thickened margins round the bones. An extension of this marginal 
cartilage traverses the interior of the bone, separating the upper from the lower osseous 
portions ; but notwithstanding this modification of the type, the essential process 
of growth in all these instances has been the same as in the previous examples ; 
chondriform bone being produeed at the two surfaces of the cartilage, and membra- 
niform bone still more externally. Thus in young states the latter constitutes two 
independent unconnected layers, separated, even in the most central portions, by a 
very thin layer of the cartilage, which is found to increase rapidly in thickness as we 
approach the periphery. In the bones of matured individuals these two layers are more 
or less united by the production of cancellated cavities and canals, which pass from 
the one to the other through the intervening cartilage. In the first instance these have 
merely been open spaces, produced by the partial absorption of the cartilage ; but they 
have soon become lined with a membrane, which, as in fig. 39 has been converted 
into bone by a calcification of its lamellse, chondriform bone being produced at the 
same time in the contiguous cartilage. I have especially observed these secondary 
bone-growths to be developed in the centre of the parietals and alisphenoids. 
In the supraoccipital, the modes of increment are the same as in the bones just 
described, only throughout the greater part of this bone there has existed a very 
marked difference in the development of the superior and inferior osseous layers. 
Whilst the former is of considerable thickness, the latter is very thin. This fact 
prepared me for the existence of a still more marked difference in the case of the 
