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shrinkage. The interior of the enamel organ is really composed of 
large irregularly round cells, -which are modifications of the cuboidal 
layer of epithelial cells. These cells are apparently very soft and 
watery ; and, being very large, the manipulation and chemical treat- 
ment necessary for mounting in balsam causes a complete evacuation 
of the cell contents and shrinkage of the intercellular substance. I 
cannot conceive that the points of intersection of the reticulum con- 
taining nuclei represent anything else than the corners of the larger 
cells which contain nuclei, and which, because of the small area, are 
not washed out in the treatment of the section. Any other view 
makes it necessary to account for the development of two totally 
distinct sets of cells from the original cuboidal layer. 
The appearances seen in sections of mature enamel confirm the 
views I have just presented, as to the methods of formation of this 
tissue. I have said that the structure of the cytoplasm of the amelo- 
blasts varies markedly. Scarcely two specimens can be found in 
which the structure of the cells is exactly alike ; but all of the 
observed differences may be broadly classed under two headings — 
cells in which the cytoplasm is arranged in sections, and others in 
which the cytoplasm is in the form of continuous parallel strings or 
fibres united by offshoots. We shall find that one or other of these 
two forms is usually to be seen as the calcific organic matrix of the 
completely formed enamel rod. 
Passing now from our study of normal or sound enamel, let us for 
a few moments examine the appearances of decaying enamel ; for I 
think we shall find that caries of this tissue throws an important 
side light on the problems we are considering. If a section of enamel 
be treated for a short time wfith lactic acid, the appearances are such as 
are shown in the photograph now on the screen. In photographs of 
decaying enamel or enamel sections treated with acids, we find the 
rods sometimes split or separated into sections by the dissolution of 
the cement substance. In other specimens, when the cement substance 
is dissolved, there remains nothing but the calcified plasmic strings, 
which also finally disappear under the continued action of the acid.^ 
In conclusion, I wish to call your attention to the bearing which 
the structure of fully formed enamel, and the relation of this structure 
to that of the cytoplasm of the ameloblast, have upon some of the more 
recent problems of cytology. There are at present, as you are all 
doubtless aware, two theories concerning the structure of protoplasm, 
one or other of which is accepted by most of the leading histologists. 
One is the foam or alveolar structure theory of Biitschli, and the 
other that advocated by Fromann, Arnold, Fleming, and probably by 
much the greater number of contemporary investigators. This view 
teaches that the more solid portions of the cell structure consist of 
coherent threads, strings, or fibres, which everywhere pervade the 
ground substance, either separately or united by offshoots, and gene- 
rally combining to form a mesh work, reticulum, or spongiose structure. 
