MUSEUM OF COMPARATIVE ZOOLOGY. 257 
Fig. 22. The same in the middle of the second incisor region. 
Fig. 23. The same in the middle of the third incisor region. 
Fig. 25a shows the appearance seven sections further back than Figure 23. 
Fig. 24 is a section behind the centre of the third incisor, showing a part of 
the dental lamina cut off from the buccal epithelium by the ingrowth of 
mesoderm. 
Fig. 25 is a section through the middle of the canine region behind the region 
where there is a narrow connection of the enamel-organ germ with the buccal 
epithelium. 
Fig. 25a. The enamel germ is divided by the deepening of a constriction 
shown in Figure 25 into two arms, the cross-sections of which appear in this 
figure as two isolated patches of epithelium. 
Figs. 26—29a are sections through the left half of the upper jaw of an embryo 
sheep 112 mm. long, which was hardened in chromic acid and stained with 
Czoker’s cochineal, followed by borax carmine. The sections were made in the 
same way as described in Figs. 1-10. They are magnified only 11 diameters. 
Fig. 26 shows the condition of the plunging wall in the region opposite the first 
incisor of the lower jaw. 
Fig. 26a shows the dental lamina more sharply marked off from the plunging 
wall as seen a few (18) sections further back than Fig. 26. 
Fig. 27 is a section through the middle of the region opposite the second in- 
cisor of the lower jaw, showing a small but well defined dental lamina. 
Fig. 28 shows the dental lamina in the region of the third incisor. 
Figs. 29, 29a, are sections through the canine region. 
Fig. 29 shows the rudimentary lamina to be still continuous with the buccal 
epithelium ; but four sections further back it is detached from the “ wall.” 
Fig. 29a.. Back of the canine region the lamina is continued as a ridge or fold 
of epithelium, which soon changes from a horizontal-to a vertical position. 
PLATE IL. 
All figures of this plate are magnified 175 diameters. 
Fig. 30. The deep portion of the dental lamina of Fig. 14. The change in 
the thickness of the malpighian layer is to be seen here, also its irregularity of 
outline. 
Fig. 51 is a highly magnified view of the incisor region shown in Fig. 24. 
The difference between the differentiated and the undifferentiated portions of the 
lamina, which are here both surrounded by mesoderm, is well marked. 
Fig. 32 gives a view of a portion of Fig. 25. The histological condition of the 
canine germ is the same as that of the differentiated portion of the dental lamina 
in Fig. 31. 
Figs. 33-35 are from cross sections of the upper jaw of an embryo sheep 
93 mm. long, which was treated with picrosulphuric acid and stained in borax- 
carmine. 
Fig. 33 is a section through the germ of the canine tooth. That portion of the 
neck of the germ which is nearest the buccal epithelium is resolved into a series 
of epithelial knots or islands. 
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