DEVELOPMENT OF THE SCALES AND BONES OF FISHES. 
669 
that I had previously applied to scale-structures. This especially consists in making- 
sections, of each bone to be examined, in every direction that appeared likely to 
reveal any new feature of its structure, and examining the same bone, as far as was 
possible, in fishes of different ages. The result has satisfied me that Professor 
Sharpey’s views not only receive the clearest possible confirmation from the study of 
the developing bones of osseous and cartilaginous fish, but that a large amount of 
additional light is thrown upon those views by such a study of the less advanced 
members of the vertebrate series. 
Dr. Sharpey has pointed out the fact that two distinct processes are put in opera- 
tion during the growth of a mammalian bone. The first is the development of a car- 
tilaginous matrix and the deposition of calcareous matter in its intercellular sub- 
stance ; the second is a deposition of similar matter in the fibrous membranes of the 
investing' periosteum and perichondrium, prolongations of which, dip[)ing into the 
interior of the organism, exhibit the same tendency to become calcified. 
Both these processes exist amongst fishes, leading by their various modifications 
to very diversified results. In a great number of examples they are both seen in 
operation in the same bone. In some of these cases, the earlier cartilaginous bone- 
growth has been re-absorbed, and its place occupied by new bone developed in true 
fibrous membrane, in a manner that resembles, though it is not absolutely identical 
with, the process which Dr. Sharpey has shown to take place in mammalian bones. 
In other instances, the two structures, so distinct both in their origin and in their 
aspect, are developed side by side, and continue to retain their respective positions 
even in the most matured animals. This is especially the case in some fishes, which, 
like the Pike, permanently retain a large amount of cartilage in connection with their 
true osseous elements. 
But in addition to these, there also exists a third group, apparently including the 
whole of the Sharks and Rays, in which only the first of these processes has gone on. 
The calcareous elements of their skeletons are nearly all constructed on the type of 
the temporary and transitional bone-growths developed in the intercellular substance 
of the cartilage along the line of ossification ; only in these cartilaginous fishes, that 
which is transitional among the higher vertebrates, is here permanent. These endo- 
skeletons are nearly all what may be termed chondriform. The two processes of 
growth being essentially different, it is desirable to apply to them distinct and in- 
telligible names, and thus avoid the necessity of employing descriptive phrases every 
time that each one is referred to : the terms chondriform and membraniform appear 
sufficient for this purpose ; the former being employed to designate those calcareous 
growths which are formed within the substance of the true cartilage, and the other 
being applied to the analogous growths which are formed by the calcification of 
laminse of fibrous membrane. I would first direct attention to some examples of 
chondriform bone, as constituting the simplest type of true osseous structure. 
Several writers have recorded the fact, that in the Plagiostomous fishes the carti- 
MDCCCLI. 4 R 
