MUSEUM OF COMPARATIVE ZOOLOGY. 255 
EXPLANATION OF FIGURES. 
The following abbreviations are used in the figures. 
enn. Canine. Ing. Tongue. 
ert. Mkl. Meckel’s Cartilage. l. ing. Linguat side. 
et, Epithelium. md. Lower jaw (and os mandi- 
g. de. Dentine germ. bularis). 
g. cnn. Enamel-organ germ of canine. ms d. Mesoderm. 
g-ena, Enamel-organ germ. mur. Plunging wall. 
Gut. < “of first incisor. Mr. Upper jaw (and os maxillaris). 
Guitar 7 s second “ n. Nerve. 
Chie o ss ehirdas o.ena. Enamel organ. 
g.pr.mol’. “ first premolar. pr. mol. Premolar. 
ts Incisor. st. con. Stratum corneum. 
la.de. Dental lamina. st. Mpg. Stratum Malpighi. 
1. lab. Labial side. 
The average thickness of the sections is about 7.5° 4, and the number of each 
section in the series to which it belongs is indicated by the number in parenthesis 
adjacent to the number of the figure. 
PLATE I. 
Figures 1-25 are magnified 17 diameters ; figures 26-29a, 11 diameters ; figures 
30-37, 175 diameters. All were drawn with the aid of the Abbe camera. 
Figures 1-10 are from sections of the left side of the jaws of an embryo sheep, 
37 mm. long, preserved with picrosulphuric acid, stained by means of Czoker’s 
cochineal followed by alcoholic borax-carmine, imbedded in paraffine and mounted 
in Canada balsam. The figures all show the anterior face of the sections, which 
were so cut that the plane of the first section was parallel to the median plane of 
the head, the object being gradually rotated so as to keep the plane of the sections 
as nearly as possible perpendicular to the outer margin of the jaw. 
Fig. 1 shows the plunging epithelial wall of the upper jaw, and the centre of 
the germ of the enamel organ of the first incisor on the lower jaw. 
Fig. 2. On the lower jaw the section passes through the region between the 
first and second incisors, where only the dental lamina is to be seen. On the 
upper jaw the plunging wall is continued into the dental lamina without there 
