38 
PROGRESS OF MICROSCOPICAL SCIENCE. 
through the capillary channels of the dentine, so that the tooth, when 
the fish is alive, is brilliantly red. The matrix of the dentine is dense 
and solid ; i. e. it is not permeated by dentinal tubes. The transition 
between typical and vaso-dentine, such as that of the Gadidae, and 
hard unvascular dentine, such as that of most mammalian teeth, is 
gradual. Thus most of the Pleuronectidae have teeth which at their 
basal halves consist of typical vaso-dentine without dentinal tubes, 
just like that of the Gadidae ; but above the middle, dentinal tubes 
radiating out from the central pulp-chamber begin to appear, at first 
sparsely, and the capillary canals to become fewer, till the apex of the 
tooth consists of ordinary fine-tubed dentine, in which few, if any, 
capillary channels exist. And in Serrasalmo there are teeth which 
are throughout composed of a dentine permeated by dentinal tubes, 
but in the basal half of the tooth a few capillary channels are present. 
From such a form of dentine to ordinary hard unvascular dentine is 
but a short step. The development of osteo-dentine is illustrated by 
a description of the teeth of a pike ; the outer layer is developed, like 
dentine, from a layer of cells analogous to, though less specialized 
than, odontoblasts ; and so soon as this has been calcified the interior 
of the tooth is formed by a rapid ossification, just as the subjacent 
bone is formed. Vaso-dentine therefore differs much less from true 
or unvascular dentine than osteo-dentine does, the relation between 
the three tissues being well seen in the teeth of Sparidse. In Sargus 
ovis the incisor-like front teeth appear to be implanted by long roots ; 
these are formed by the dentinal formative pulps, just as are the roots 
of ordinary rooted teeth. But there is this peculiarity in the nature 
of the process : the dentinal pulp, so long as the “ crown ” (or portion 
which will be above the bone) is being developed, is converted into 
fine-tubed unvascular dentine; but so soon as the root or implanted 
portion commences to be formed, this same dentinal pulp, the apex 
of which is even yet forming unvascular dentine, calcifies into vaso- 
dentine. Without there being any exact break or breach of continuity, 
the change from true dentine to vaso-dentine is sudden, and the tooth 
is easily broken off at this point. When the greater part of the 
length of the root has been formed, the manner of calcification again 
changes, this time not so abruptly, till near to the end of the root the 
dentinal pulp becomes converted into osteo-dentine, which is quite in- 
distinguishable from and blends insensibly with the surrounding coarse 
bone by which the tooth is fastened into the socket ; there is, in fact, 
no reason for calling it anything else than coarse bone, except the fact 
that it is the product of calcification of a dentinal pulp. In this case 
a single dentinal pulp forms first hard dentine, secondly vaso-dentine, 
and at last osteo-dentine. Another variety of complex dentine is 
brought about by foldings and subdivisions of the formative pulp : 
both vaso-dentine and osteo-dentine are formed by the calcification of 
simple pulps ; but in many instances the odontoblast-bearing surface 
of the pulp is itself complicated in form, and a dentine arranged as it 
were round many pulp-chambers is the result. For this no better 
name than plici-dentine (also a term already in use) suggests itself : 
it is to be seen in its simpler form at the base of the teeth of Lepi- 
