OF THE NEWT, FEOGr, SLOWWOEM, AND GKREEN LIZAEI). 
289 
a capsule, as happens in a mammalian tooth-sac ; and although the fibres of the connec- 
tive tissue are to some slight extent pushed on one side, so as to be in some measure 
concentrically ranged round the growing tooth-sac, yet they do not form any thing like 
a definite investment to it. Vessels are abundant in the immediate neighbourhood of 
the tooth-sacs ; but they do not appear to enter them, save when the tooth is somewdiat 
advanced. 
Although I have never been fortunate enough to obtain a specimen in which the 
first tooth-sacs were in process of formation*, yet, owing to the very large number of 
successional teeth which are formed, it is possible to trace out all the stages of the 
process in an adult animal. 
The processes of epithelium which are to be found on the inner side of the youngest 
tooth-sacs have already been mentioned ; they are very well seen in figs. 2 & 5 (f&fi) '• 
thus in fig. 5 we have three stages in the formation of a tooth-sac — namely, the earliest 
dipping down of epithelium, as seen at f[, and an epithelial process which has reached 
down nearly to the base of the area of tooth-development, while to the right of this is 
a fully formed tooth-sac, which, however, still retains its connexion with the epithelial 
cells above it. 
These epithelial processes, shooting down from the surface into the connective 
tissue beldw, which they push out of their way, are clearly homologous with the 
“ enamel-germs ” of mammalian teeth ; and just as the enamel-germ of a human per- 
manent tooth is given off from the neck of cells which connects the enamel-organ of 
the deciduous tooth-sac with the oral epithelium, so in these Batrachian teeth the 
enamel-germs of the successional teeth are given off from those of their predecessors f 
(see figs. 5 & 7). 
AVhen the end of the epithelial process has nearly reached to the base of the area of 
tooth-formation, its cells become more distinctly columnar in character, and its end 
enlarges, so that it has a spherical form when viewed on its surface ; but seen in section 
it presents the appearance shown in fig. 6, in which the extremity of the enamel-germ 
has assumed the form of a bell-shaped cap, embracing the dentine-papilla inside it. At 
this early period the cells of the enamel-germ next to the dentine-papilla are elongated ; 
and the dentine-papilla shows indications of the bicuspidate form of the crown in one 
of my sections (fig. 8), though this may perhaps be accidental, as I have not seen it 
constantly. 
A peculiarity in the appearance of the tooth-sacs of the newt is that they are very 
* Professor Huxley informs me that tooth-development in the newt commences at a very much earlier 
period than in the frog. 
f This reopens the question, are the milk or the permanent teeth of diphyodonts homologous with the single 
set of monophyodonts ? — a question which appeared to have been set at rest by Professor Flower’s paper 
(Journal of Anatomy and Physiology, 1869). The arguments in that paper appeared to he conclusive in favour 
of the view that the milk-dentition was the thing superadded; hut this is difficult -vto reconcile with the deve- 
lopmental relation existing between tooth-germs of the two. 
