37 
Development oe vascular dentine. 
connected with one another by cross branches. The canals of the completed tooth 
are the spaces left out by and included between these tracts of calcification ; the 
contents of the canals are the masses of pulp so enclosed. 
If the calcifying’ dentine of one of the Gadidse be compared with this of the Pike, 
it will be seen that the internal surface of the former to which additions are being 
made is smooth and of even outline (see fig. 4 and fig. 9), whereas the internal 
surface of the latter is extended out into a large number of greatly elongated 
processes. This is well seen in the transverse section represented in fig. 16*. In 
fig. 16 the ends of one or two of the processes to which additions are being made 
are represented. The growing surfaces are found to be covered with a layer of nearly 
spherical cells, like the osteoblasts by which bone is built up ; but there are no 
elongated cells, such as are ordinarily called odontoblasts, to be found. In fact, 
viewing the tooth of the Pike from the point of the minute development of its 
constituent parts, it might not inaptly be described as consisting of a core of 
porous bone coated over with a thin skin of dentine, and, had not the name 
osteo-dentine been somewhat loosely applied to other tissues than this, no more 
fitting term by which to designate it could have been found. The teeth of those 
Plagiostomi which have been examined by Hertwig and by myself are developed 
in a manner precisely similar, as might be expected from their structure. 
Thus far I have sought to distinguish from amongst the tissues hitherto indis- 
criminately classed as vaso -dentine two strongly marked varieties. 
(1.) A dentine formed wholly by calcification of a layer of special odontoblast cells, 
and permeated by a ‘system of canals formed around and enclosing capillary blood- 
vessels. 
To this variety I would apply, and to it strictly limit, the application of the term 
Vaso-dentine. 
(2.) A dentine formed at its surface only by a cellular layer, its interior being 
formed by the extension of calcifying trabeculae through the substance of the pulp. It 
also is permeated by larger channels, but these bear no relation to capillary blood- 
. vessels. 
To this variety I would limit the application of the term Osteo-dentine. 
There is at least one other way in which the structure of dentine may become 
complicated, and in which it may come to contain a system of channels larger than 
dentinal tubes. The formative pulps of the two varieties of dentine to which I limit 
the terms vaso-dentine and osteo-dentine are simple cones, but the surfaces of the 
pulps themselves may be complicated by various folds and inflections, so that by their 
calcification a complicated looking tissue results. 
As an example of this I have figured the transverse section of a tooth of Lepidosteus 
osseus , which I was so fortunate as to procure in chromic acid solution (fig. 12). 
In this the complication of the tissues is due to complication in form of the formative 
pulps ; the whole of the dentine is formed by the calcification of odontoblasts, thus 
