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. 23a 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 oif 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 tliird 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 II. 



AH 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. 31 is a highly magnified view of the incisor region shown in Fig. 24. 

 The difference between the diiferentiated 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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