DEVELOPMENT OF THE SCALES AND BONES OF FISHES. 
683 
process^ which connects the epityrapanic with the upper extremity of the pre-opercular 
bone. As the young fish increases in size, the central portion of this cartilage be- 
comes absorbed, and is replaced by large cancellated cavities which are subsequently 
filled with cells of adipose tissue. 
Fig. 38 represents a vertical section of the extremity of the opercular condyle as it 
exists in a Pike of about 3 lbs. weight. All the essential phenomena which are seen 
in the carpal bone are repeated here, only in a still more obvious form. At 38 a the 
cartilage-cells are distributed in small detached groups. At 38 h their arrangement 
is altered ; they are disposed in interrupted vertical rows, passing from side to side, 
at right angles to the ossifying surface : a disposition which maintains throughout 
that portion of the cartilage which is invested by bone. At 38 c there is a layer of 
chondriform bone, produced in the same way as that seen in the corresponding por- 
tions of the carpal ; only here it is much more extensively developed than in that 
example. It exhibits an areolar structure, which resembles, in the closest possible 
manner, some of the forms existing amongst the exclusively chondriform bones of 
the Sharks and Rays. This tissue is surmounted by suecessive lamellm of membra- 
niform bone, 38 cl, exhibiting various arrangements of Haversian canals, 38 e. Some- 
times these canals exist as large cavities left between two contiguous lamellae, being 
the result of their mutual divergent inflections, and resembling those seen in the scale 
of the Sturgeon and the Holopty chins. At others they are the result of apertures left in 
the successive lamellae, which have not been developed at the points along which the 
canal w'as intended to pass. This especially applies to the long oblique canals which 
proceed from the interior towards the surface of the bone. The membraniforrn 
lamellae (38 f/) are obviously growths successively added to the exterior; they en- 
croach more and more upon the perichondrium surrounding the extremity of the 
cartilage as each new addition is made, being usually a little in advance of the sub- 
jacent chondriform bone. The latter circumstance we have already observed in the 
stylo-hyal and carpal bones. 
Fig. 39 represents a similar section of the proximal half of the opposite condyle (or 
that which articulates with the postfrontal) from a young Pike weighing about six 
ounces. At 39 a we have the usual internal cartilage. At 39 h there is a very thin 
layer of chondriform bone, surrounded by a much thicker cylinder of membraniforrn 
bone (39 c) composed as usual of numerous parallel lamellse. The chondriform layer 
is very thin in this instance, as compared with that of fig. 38 . I have observed that 
it appears to be developed with greater rapidity at the extremities of these processes 
than at their bases, though it exists at both points. Fig. 39 d is part of the base of the 
external process extending from the centre of the epitympanic towards the preoper- 
cular. It consists of variously inflected layers of membraniforrn bone, perforated by 
innumerable irregular Haversian canals, each of which has been formed in the way 
already described, by the omission of the portions of the successively added lamellae, 
opposite the pre-existing orifices of the canals. The internal cartilage has not been 
