520 J. T. WILSON AND J. P. HILL. 
result of the differentiation of the anterior part of mz; the 
hinder part of the tooth is not yet constricted off from the 
parent lamina. 
Stage XI.—P. nasuta: pouch specimen. 
Length from tip of snout to root of tail : . 127 mm. 
Length of skull . : ‘ ‘ ~ AS Ss 
This stage is only slightly, if at all, older than Stage x. 
Indeed, in certain respects development is less advanced, for in 
the upper jaw no teeth have yet broken through the gum, 
though the canine and premolars are causing very marked 
projection of the latter. 
Both the deciduous premolar and the Anlage of p3 closely 
resemble those of the preceding stage. The proximal part 
(or “‘neck”’) of the residual lamina, where that forms the 
Anlage of p3, offers a very striking example of cell-nest for- 
mation as a comcomitant of the process of disintegration of 
the lamina. Here we have the papillated Anlage of p2 borne 
upon an attenuated lamina, whose proximal border towards 
the oral epithelium exhibits a number of large globular epithe- 
lial swellings showing the concentric arrangement of the con- 
stituent cells characteristic of epithelial “ pearl ” formation. 
The molars resemble those in Stage x. 
In the lower jaw diz is in the same degenerate condition as 
in Stage x, but the dentinal nodule representing it is even 
smaller. Otherwise the lower jaw presents no noteworthy 
features. 
Stage XII.—P. obesula: pouch specimen. 
Length from tip of snout to root of tail ‘ . 120 mm. 
Length of skull . : : 5 s Agen. 
The lower jaw only of this specimen was studied. Though 
its dimensions do not exceed those of Stage x1, it represents 
a decidedly older stage. This is explainable by difference in 
specific characters. 
The Anlage of pz still shows a rudimentary papilla project- 
ing into the slightly cupped enamel-germ. The whole Anlage 
