566 J. T. WILSON AND J. P. HILL. 
labially, nearly at right angles with the stem, as shown in figs. 
30 and 81 (/. 0.) and in Woodward’s figs. 25 aand 25 6. 
The definiteness, constancy, and longitudinal extension of 
this form of labial projection markedly distinguish it from 
those other kinds of outgrowths on the labial side of the dental 
lamina to which we have alluded above. 
No satisfactory explanation of the meaning of this structure 
seemed forthcoming until, after some incidental study of serial 
sections of pig embryos, the conclusion was forced upon us 
that this secondary labial laminar projection really represented 
the continuation of the labio-dental, or, better, labio-alveolar 
epithelial lamina (precursor of the lip-furrow or labio-alveolar 
groove) which the researches of Rose (9) in the human teeth 
have shown to arise from a common Anlage with the dental 
lamina proper. 
This identification seems to us so important and so unex- 
pected, as applied to structures so far back in the molar region, 
and especially in marsupial animals (in which, at least ante- 
riorly, the lip-gum furrow is long suppressed), that we find it 
necessary to exhibit in some detail the facts upon which our 
conclusion is based. 
It must be admitted that the explanation offered seems far- 
fetched and improbable, nor do figs. 830 and 81 seem to lend 
much countenance to it. Nevertheless we hope to show good 
reason for the belief that the labial laminar outgrowth is, at 
least, an epithelial lamina which is largely independent of the 
dental lamina proper. 
In the first place we must state that in respect of the mode 
of extension backwards of the dental lamina in Marsupials, 
our observations are so far at variance with those of Rése 
upon human molar development. There can be no doubt that 
in the animals examined by us the greater part of the molar 
lamina grows posteriorly by extending backwards its con- 
tinuity with the oral epithelium, and not merely by extending 
back freely into the mesoderm beneath the epithelium as Rése 
has described it. The anterior, greater part of it appears to 
be as much the product of direct ingrowth from the oral 
