MUSEUM OF COMPARATIVE ZOOLOGY. 



255 



EXPLANATION OF FIGURES. 



The following abbreviations are used in the figures. 



The average thickness of the sections is about 7.5 fi, 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 epitlielial 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 



