116 THE MICROSCOPICAL NEWS. 
This first assumes a spherical form, but it soon changes in the 
course of development into a cap-like investment to the dentine 
bulb, and is thenceforth modelled by the latter into whatever shape 
it assumes, being thickest over the apex of the bulb and thinning 
down as it approaches the base. In all probability, the enamel is 
formed by the actual conversion of the enamel cells. Almost 
simultaneously there arises as an opaque point in the submucous 
tissue, but in close proximity to the enamel organ, the dentine 
bulb. This soon begins to assume the form of the apex of the 
fyture tooth, becoming simply conical if a canine, but if it is to be 
a tooth with more than one cusp, sending as many conelike 
depressions into the enamel organ, and occupying these depres- 
sions, as there are points in the new tooth. Coincidently with 
these changes in the dentine bulb, the layer of cells forming its 
surface, which is now in close relation with the enamel cells, be- 
comes differentiated from the parts beneath it and forms the odon- 
toblasts, or the cells which become dentine by calcification. This 
process begins at the points of each of these cones, and progresses 
until they coalesce, as may be seen in this partly-formed monkey’s 
tooth. Before calcification can take place, the dentine bulb is 
metamorphosed into the dentinal matrix, or tooth cartilage. 
This organ assumes just the shape and size of the dentine, and 
can easily be obtained in the formed tooth by dissolving out the 
salts of the dentine by means of hydrochloric acid, which will leave 
the matrix complete. The walls of the dentinal tubes, called the 
dentinal sheaths, are formed in and are a part of this matrix ; and if 
the process of its decalcification be carried far enough, it can be so 
much destroyed by the acid that the sheaths alone will remain as 
a transparent slime. This demonstrates not only that the dentinal 
tubes have definite walls, but that they are singularly indestructible. 
Indeed, they are so persistent as to be found in fossil teeth. It is 
around the sheaths of these walls that the’ calcification of the 
matrix takes place, and thereby are formed the tubes seen in the 
sections I have prepared for the microscope. Simultaneously 
with the formation of the dentinal matrix, its calcification com- 
mences ; and also simultaneously with the general metamorphosis 
of the dentine bulb, is formed the dentinal fibrils. These fibrils 
are an extension of the pulp into the tubes, and are little else than 
the unaltered protoplasm of the odontoblast cells,—that is, the 
tooth-germ ceils. Thus we have three stages in the conversion of 
the bulb going on at once. First, there are the dentinal fibrils. 
Second, there is forming the dentinal sheath to enclose the fibrils 
and as walls to the tubes, a substance which lies just on the border- 
land of calcification, if it really does not calcify to some extent, 
which I think it does. And third, there is the calcifying matrix, 
which fills all the space between the sheaths. The dentine begins 
