of the Tnnfh-7?uf?tmenfs in Rodents. 321 



twenty-six times, about nine sections (15 /x) further back. 

 The section j)asses tlirough the anterior circumfcrerice of the 

 orifices of Stenson's canals. Of the enamel-bell of the 

 anterior rudiment only the posterior margin is still cut, while 

 inwards from it is seen the dental fold detached from the 

 e|)ithelium. 



Fig. 9 lies three sections further backwards (same scale). 

 The section passes through the narrow interval between the 

 two rudimentary enamel-organs The dental fold is to be 

 observed, and to the side of it a thickening of the epithelium 

 of the oral cavity, with which the fold at other spots is con- 

 nected. Lastly, fig. ] lies only two sections behind the fore- 

 going, and shows the second rudimentary enamel-bell. This 

 is by far less distinctly characterized as such than is the 

 former one. For, in the first place, it is directly attached to 

 the outside of the epithelial fold, und is united to the epithe- 

 lium of the oral cavity by means of a short thick connective 

 cord. Secondly, the enamel-pulp is absent from its interior, 

 and, moreover, the connective-tissue-pulp, or tooth-papilla, is 

 not so thickly nucleated as in the case of the anterior rudi- 

 ment. Yet the whole structure is enveloped in a concentric 

 membrane of connective tissue (not visible in the figure), and, 

 especially after examination of the model, fig. 14, it is hardly 

 possible to doubt that tiie rudiment is of the nature of an 

 enamel-organ. 



Cavia. 



Of Cavi'a I was able to examine a small specimen of the 

 cephalic length of 4'5 millim. This presented no special 

 interest for our subject, since the tooth-rudiments were still at 

 the very commencement of their development*. 



Then come two series, one through the head of an embryo 

 measuring r4 centim. in cephalic length and 2*9 centim. in 

 all, while the other was derived from a somewhat younger 

 specimen, which Prof. Froriep, of Tubingen, was kind enough 

 to make over to me. The stages were so far very favourable in 

 that they corresponded to those of the rabbit in which the rudi- 

 mentary tooth-germs are most distinct. The result of the 

 investigation was, however, an almost negative one : no trace 

 of a rudimentary tooth and no trace of a germ for the second 

 incisor. In the diastema tliere were found here and there 

 thick cones of epithelium, which projected downwards for a 



* I am indebted for this specimen to the kindness of Dr. Keibel, of 

 Freiburg i. B., vrho placed at my disposal a whole series of guinea-pig 

 embryos. Most of these, however, were too yoimg and consequently 

 not available for the purposes of my investigation. 



