4.52 J. T. WILSON AND J. P. HILL. 
slightly deeper and opener, the ingrowth at the same time 
becoming thicker. Finally the groove opens out and dis- 
appears, leaving only the thickened ingrowth, again lenticular 
on cross-section, which in turn disappears a few sections further 
back. 
Rose’s fig. 1 (1) sufficiently illustrates the condition found 
by us in this early stage. 
Stage IJ.—P. nasuta: mammary fetus. 
Length from vertex to root of tail . : . 17 mn. 
Head length . : ; ‘ Tor as 
Transverse (coronal) and sagittal series studied. 
The dental lamina is already pretty fully developed, though 
the differentiation from it of enamel-organs has not progressed 
very far, so that only a very few enamel-organs can be recog- 
nised as such, and there are relatively long stretches of quite 
undifferentiated lamina. The latter is relatively thick (fig. 
1, dl.), especially at its so-called “neck.” In other words, 
the groove in the mesoderm occupied by the elongated mass 
of ectodermal cells forming the lamina is a widely open one, 
its lips not yet being approximated so as to thin out the 
lamina towards its connection with the deep surface of the 
oral epithelium. 
Upper Jaw.—In the upper jaw the dental lamina extends 
anteriorly almost, but not quite, to the mesial plane: mesially 
it is separated by an interval from the lamina of the opposite 
side. In this anterior incisor region the structure is best 
studied in sagittal sections, for in the sagittal plane we obtain 
nearly true transverse sections of the lamina for some distance 
outwards. Fig. 6 shows a sagittal section parallel with the 
mesial plane which just shaves the margin of the orifice of 
Stenson’s duct (S.d.), and cuts the dental lamina (di/.) trans- 
versely to its axis some distance in front of this. The lamina 
is seen to be surrounded by young connective tissue, and the 
whole is capped by an imperfect dome-shaped roof of bone— 
the commencing alveolar portion of the premaxilla (pmz.). 
This dome-shaped roof in turn causes an upward inflection of 
