518 J. T. WILSON AND J. ‘P. HILL. 
are here and there discernible by the lingual side of the teeth 
as in the canine region, but on a smaller and less impressive 
scale. Here and there the form of the residual lamina (pear- 
shaped, with long stalk) is preserved, but for the most part the 
vestiges—generally absent altogether—are in the form of scat- 
tered minute epithelial cell-groups. 
The deciduous premolar is in the act of cutting the gum with 
the most anterior part of its crown. 
The process of eruption is very clearly seen to be attended 
by the flattening out of the enamel epithelium covering the 
tooth crown and its conversion into squamous epithelium with 
concomitant formation in it of epithelial “ nests’ or “pearls” 
close to the tooth cusps. This nest-formation also proceeds in 
the oral epithelium over the tooth. Eventually the epithelial 
pearls become placed quite near the surface, and they then 
undergo rupture and disintegration, their cores of concentric 
epithelial cells being lost and the tooth crown exposed. These 
different stages can be plainly followed in this one case by 
tracing dp? backwards from its point of actual eruption to- 
wards the posterior more deeply placed part of the tooth, where 
the preparations for eruption are proceeding. The cell-nests 
here noted as forming in the enamel epithelium are in all pro- 
bability similar to those epithelial nodules noted by Poulton 
(25) as appearing in the “ most superficial part of the middle 
membrane of the enamel-organ immediately over the apex of 
each chief cusp of the large broad posterior tooth” of Orni- 
thorhynchus (see his pl. ii, figs. 4,5, 11, and 12, and pl. 
ili, fig. 6). It may be noted, however, that in this stage of 
Perameles the tooth is very much more advanced than those 
figured by Poulton in Ornithorhynchus, and the middle 
stratum of the enamel epithelium can no longer be distin- 
guished as such. All we can say is that in the midst of the 
epithelial investment of the tooth-crown, formed by the now 
flattened enamel epithelium (as well as in the neighbouring 
oral epithelium), these cell-nests are formed, and that the 
process of eruption of the tooth under notice is accompanied 
and so far conditioned by their denudation and disintegration. 
