DEVELOPMENT OF THE SCALES AND BONES OF FISHES. 
697 
Saw-fish and numerous other Plagiostomes, as well as of the scale of the Siluroid, we 
have what closely resembles ordinary dentine, if not its homolog-ue. In the elaborate 
structures covering the scales and occupying the interior of the spines of Ostradon 
and of Coelorhynchusy we have beautiful examples of osteo- and vaso-dentine. The 
tubes which enter from the external surface of the same structures, remind us again 
of the very similar tubes, which, as Mr. Tomes has demonstrated, penetrate in like 
manner the enamel of the teeth of most marsupial animals. The many varieties of 
kosmine which I already possess, leaves little doubt on my own mind that when we 
are as well acquainted with the numerous modifications of this tissue as we already 
are with those of dentine, it will be found that every variety of the latter has its re- 
presentative amongst the former class of structures. When we remember the un- 
doubted fact that all these forms of kosmine have been produced by the calcification, 
not of a cellular pulp, like that to which the growth of dentine has been attributed, 
but of successively added lamellae of purely fibrous membrane, we are bound to admit 
xhQ possihility that dentine may have been formed in the same way. So long as the 
old opinions respecting the direct influence of cells on the production of the Haversian 
systems of mammalian bones were recognized, nothing was more natural than to 
account for the growth of teeth by a similar process ; especially since a cellular 
structure, to which to refer the process, was so conveniently at hand. But now that 
the growth of bone is almost universally regarded by intelligent physiologists in 
a different light, and bearing in mind that a series of tissues closely resembling 
dentine have been formed without the intervention of any pulp-cells, the question 
naturally suggests itself, Mdiether the cells of the pulp have any direct connection 
with the calcification of a human tooth. I have already pointed out that the “ Pulp- 
cavity” of the spine of Ostradon, the recent representative of the fossil Coelorhyndius, 
is chiefly occupied by a reticular fibrous membrane, such as is seen in the scales of 
the same fish. The dermal defence bones of the common Picked Dog-fish {Spinax 
acanthias, Cuv.) contain a “ pulp” of true cartilage, exactly like that with which the 
bone of the endoskeleton is associated. Both these tissues, existing in the interiors 
of structures which bear a very close resemblance to true teeth, are very different 
from the pulp-cells of a mammalian tooth. 
There are some points in the structure of a mammalian tooth, which suggest a 
doubt as to the correctness of the pre-existing hypotheses. 
In Dr. Muller’s Physiology, a representation is given of the external portions of 
some of the dentinal tubes, from the incisor of a full-grown Horse (Dr. Baly’s Trans- 
lation, 2nd edition, p. 428, fig. 41), in which many of the lateral twigs of each tube 
terminate in well-defined lacunae with numerous canaliculi ; a dense series of similar 
lacunae is also represented as existing at the extremities of the main tubes, immediately 
below the enamel. I have carefully examined this portion of the tooth, both in the Horse 
and Ox, and, with Professor Owen, I differ from the interpretation given by Di-.Muller. 
I am disposed to think that the dentinal portion immediately subjacent to the ex- 
