458 J. T. WILSON AND J. P. HIM. 
at the present stage has arisen by direct proliferation and in- 
growth of the cells of the Malpighian layer of the oral epithe- 
lium, as is the case in front of the molar region. 
An examination of later stages proves that the line of conti- 
nuity between the oral epithelium and the molar lamina grows 
backwards, for some time, pari passu with that rearward 
progress of the lamina itself which accompanies the gradual 
elongation of the jaw. 
At the present stage the entire dental lamina has retained 
its connection with the oral epithelium. 
In a very young mammary foetus of Dasyurus the molar 
dental lamina does not end abruptly, but, after somewhat 
suddenly becoming shallower, undergoes a further very gradual 
diminution in depth, until it fades away into a thickening 
of epithelium, lens-shaped in cross-section, like that which 
has been described by Rose as constituting the earliest trace 
of the dental lamina in the anterior region of the jaw. 
Lower Jaw.—The armature of epithelium on the oral 
surface of the lower jaw near its tip is only moderately thick, 
and at first it thins slightly as it is traced backwards; then 
suddenly on each side a localised thickening of epithelial cells 
appears, which invades the underlying connective tissue, and 
suggests the character of an extremely thick, broad, and some- 
what shallow dental lamina. In reality, however, the definitive 
dental lamina appears abruptly a short distance behind as a 
further downgrowth from the thick cell-mass in question. 
(The precise significance of the latter is doubtful. As it is 
traced further back it becomes largely continuous with the 
corresponding mass of the opposite side. Further back still, 
the fused structure is seen to be absolutely continuous with 
the epithelial cell-mass forming the tip of the tongue, which 
is thus firmly glued to the upper surface of the lower jaw. As 
the tongue is traced backwards it gradually frees itself from 
the latter, and the thickening of the jaw epithelium is then 
seen to have largely disappeared.) The dental lamina itself, 
springing abruptly as aforesaid from the deep surface of the 
epithelial armature of the jaw, becomes almost at once some- 
