DEVELOPMENT OF THE TEETH OF OPHIDIA. 
299 
young tooth-germs (cf. fig. 4) the outer or reflected layer of cells may he recognized ; but 
their identity is soon lost, and nothing but the “ enamel cells ” can be distinctly made 
out. They become shorter after the enamel layer has been formed (the thin coat of 
enamel has of course disappeared from the specimens figured, which are all decalcified 
sections ; had the layer been cementum it would not have done so), but do not wholly 
disappear. 
Of the. capsule, such as it is, there is little to be said; it is merely a very slight 
condensation of the surrounding connective tissue. 
As the tooth approaches completion, there is a peculiarity in the form which its 
base assumes which I have not noticed in other animals — namely, that the dentine at 
the widely open base of the tooth is often abruptly bent inwards, as though the base of 
the tooth were about to be closed by a sort of operculum of dentine (see fig. 5). 
An early germ is represented in fig. 4, measuring in its total length xxo' °f an inch ; 
it differs from a mammalian tooth-germ by its elongated form, and by the fact that the 
two layers of cells which necessarily result from the manner of formation of enamel- 
organs, namely the outer and inner epithelia of the enamel-organ, are so closely in 
contact as to be indistinguishable except at the base, there being no intermediate stellate 
tissue ; while from the tooth-germs of Batrachia and Sauria it differs in no respect save 
its exceedingly elongated shape. A still earlier stage, when the tooth-germs may be 
said to consist solely of a preparation for the formation of an enamel-organ' in the shape 
of a csecal process of epithelial cells, is shown at e in fig. 3. 
But it is not in the structure nor in the development of individual tooth-germs that 
the Ophidia are peculiar ; it is in the relation of these to one another and in their large 
number. Including the tooth which is in situ , no less than eight different stages may 
often be seen in a single section ; and their large number necessitates a peculiarity in their 
arrangement, for, remembering the small size of the tooth-bearing bones and the extreme 
dilatability of the snake’s mouth, it would be manifestly impossible that the successional 
teeth should be arranged in linear series from without inwards. 
Accordingly we find the greater number of the forming teeth to be placed nearly 
vertically one above the other, parallel with the jaw-bone and the tooth in place, thus 
interfering but little with the mobility of the mucous membrane and the dilatability 
of the mouth (figs. 2 & 3). 
The tooth next in order of succession, however, has moved inwards in a curvilinear 
direction, so that it no longer stands above the younger teeth, but lies in a measure 
between the topmost developing tooth and the one already in place. In other words, a 
tooth-germ as it progresses from being the youngest of the series to being the oldest, 
moves at first upwards, then outwards towards the teeth in use, and then again a little 
downwards, so that it describes a curved path. And not only does the growing tooth- 
germ thus bodily migrate, but it also undergoes a change in the direction of its long axis. 
Starting at the bottom of the area of tooth-development (1 in fig. 2 & e in fig. 3), its 
long axis is nearly perpendicular to that of the jaw ; but as it becomes larger it becomes 
