DEVELOPMENT OF THE TEETH OF FISHES. 
263 
The general relationship of the dental germs to the oral epithelium, to the already 
formed teeth, and to the bone is shown in the perch in fig. 4, and in the sharp-nosed eel 
in fig. 6 ; the process in the two fish is so closely similar that it seems unnecessary to 
separately describe their tooth-germs. 
An early stage is represented in fig. 7, in which the inward-growing epithelial 
process has formed an embracing cap or enamel-organ, but no calcification has as yet 
occurred. The enamel-cells are of very large size over the apex of the dentine-papilla, 
measuring about of an inch ; but at a somewhat later stage (fig. 8) the large 
enamel-cells are seen to be confined to the immediate summit of the tooth, those a 
little further down (e' in the figure) being very much smaller (two o of an inch). This 
same point is shown in the tooth-sac of a perch (fig. 5), in which the cells of the 
enamel-organ over the summit are large, but those extending down the sides of the 
tooth small and comparatively inconspicuous. 
At a still later stage (fig. 9) the large cells on the apex have disappeared, and the 
whole enamel-organ consists of the smaller, cells upon the sides of the dentine-cap, 
which are about two o 1° ttoo" °f an inch in length. 
This peculiar configuration of the enamel-organ has relation to the partial disposition 
of the enamel upon the tooth; thus the tooth of the sharp-nosed eel (fig. 10) has a 
sharp conical cap of enamel upon its summit, the enamel being absent, or so thin that I 
cannot be sure of its existence, upon the sides of the tooth*. 
We may thus say that we have exemplified within the boundaries of a single tooth- 
sac a functional and a rudimentary enamel-organ ; and it is interesting to observe that 
the appearance of the rudimentary or lower portion of the enamel-organ closely 
resembles that of the entire rudimentary enamel-organ of the armadillo, which I have 
elsewhere described (Quart. Journ. Micros. Sci. 1874). 
The fact that the development of the “ enamel-cells ” bears a direct relation to the 
thickness of the enamel to be formed also lends support to the view (which on other 
grounds I am strongly inclined to support) that the enamel is formed by the direct con- 
version of the enamel-cells into the hard tissue. 
It is difficult otherwise to account for the contrast presented by the length of the 
cells in the different parts of the enamel-organ ; and I may add that the aggregate 
length of the cells and the enamel cap already formed in fig. 8 just makes up the 
thickness of the enamel cap upon an average completed tooth. 
* The existence of terminal caps of enamel, a character upon which alone Professor Owen has founded his 
fossil genus “ Ganacroclus,” is apparently by no means uncommon ; I have found them in the eel, the perch, 
the newt, and the salamander ; and as they are very easily lost in making sections, it is probable that they are 
present upon many teeth on which their presence has not as yet been demonstrated. 
The positive determination of the absence of a thin layer of enamel upon the exterior of a tooth is a matter 
of no little difficulty, although it might at first sight appear easy enough : the double contour which is due to 
the thickness of the section can hardly be with certainty distinguished from a very thin structureless external 
layer, though the use of the dark-ground illumination will often bring the enamel layer out distinctly, owing to 
the difference in its refractive index. However, as I find the presence of an enamel-organ to be universal, the 
presence or absence of a merely rudimentary deposit of enamel ceases to be a fact of so much importance. 
