256 BULLETIN OF THE 



being any well-marked division between the two. They have a slightly different 

 direction. 



Fig. 3. A section through the middle of the region of the second incisor of 

 the lower jaw. On the upper jaw the dotted line shows approximately the 

 outline of the plunging wall of Fig. 1, as it would appear when projected on to 

 the plane of this section. The portion beyond the dotted line is the dental 

 lamina. 



Fig. 4 is from a section about midway between the second and third incisors of 

 the lower jaw. 



Fig. 5 passes through the middle of the third incisor of the lower jaw. 

 Fig. 6 is from the region between the third incisor and canine tooth. 

 Fig. 7 shows the enamel germ of the canine tooth on the lower jaw. On the 

 upper jaw the dental lamina can be distinguished from the plunging wall by its 

 size and direction, as in the preceding figures. 



Fig. 8. This section passes through the region between the canine tooth and 

 the first premolar on the lower jaw. The dental lamina of the lower jaw no 

 longer appears as an outgrowth from the epithelium of the plunging wall. The 

 dental lamina of the upper jaw is much smaller than in the preceding sections. 



Fig. 9 shows the middle of the enamel germ of the first premolar tooth of both 

 jaws. The asterisk marks the position of a longitudinal ridge of epithelium near 

 the base of the dental lamina. 



Fig. 10. A section from the region between the first and second premolars. 

 The dental lamina is pear-shaped in section throughout this region. The lateral 

 ridges at the base of the dental laminae are also continued through the greater 

 part of the region. 



Figs. 11-20 are sections througli the left side of both jaws of a sheep embryo 

 56 mm. long, which was treated in the same manner as tlie embryo of 37 mm., 

 with the exception that it was stained in Grenadier's borax carmine only. The 

 sections were cut in the same manner as in the figures just described. 



Figs. 11-15 are sections through the region of the first incisor of the lower jaw, 

 showing the enamel germ of the first incisor as it appears in different regions. 

 On the upper jaw the dental lamina and plunging wall are seen. Fig. 14 is 

 through the centre of the enamel organ of the incisor. 



Fig. 16 passes between the first and second incisors on the lower jaw. The 

 dental lamina of the upper jaw has become broader, and its section therefore 

 appears more elongated. 



Fig. 17 passes through the middle of the second incisor on the lower jaw. 

 Fig. 18 shows the dental lamina in the regions betweea the second and third 

 incisors. 



Fig. 19. This section is taken through the centre of the enamel germ of the 

 third incisor of tlie lower jaw. 



Fig. 20 is through the middle of the enamel germ of the canine tooth of 

 both jaws. 



Figs. 21-25a are sections of the upper jaw of a sheep embryo 87 mm. long, 

 treated in the same manner as the embryo 37 mm. long. The figures show the 

 anterior faces of the sections through the upper jaw of the right side. 



Fig. 21 shows the extent of the plunging wall and the dental lamina in a region 

 opposite the middle of the first incisor of the lower jaw. 



