290 Dr. W. Kiikentliiil on the 



become disposed in closer concentric strands, and forms the 

 earliest rudiment of the dental sac. The rudiment of the 

 canine tooth is considerably larger ; in the free end of the 

 enamel-organ there is a slight indentation corresponding to 

 the rudiment of the dental papilla, which is beginning to 

 appear, and which is recognizable as a number of close- 

 ])acked cells. The first and second premolars are scarcely 

 distinguishable from the dental fold, while the third pre- 

 molar, which comes next to them, is the most developed of all 

 teeth {vide fig. 1). The enamel- organ has assumed a cap- 

 shaped form simultaneously with the commencement of the 

 development of the dental papilla. The inner epithelium (the 

 enamel-membrane) exhibits the typical form of the long 

 columnar cells, and the enamel-pulp likewise begins to 

 develop. Internally to the wall of the jaw the enamel-organ 

 becomes indented by an ingrowth of connective tissue and 

 assumes a lobate form. Another series of sections from an 

 embryo 1 centim. in length, the development of wiiich is 

 slightly more advanced, shows how the ingrowth of connec- 

 tive tissue produces further back a complete separation of the 

 inner epithelial knob from the outer one, the original enamel- 

 organ. The lobe which is thus constricted oft' can only be 

 regarded as the earliest rudiment of the enamel-organ of the 

 successional tooth. 



Shortly after this the dental fold conies to an end, without 

 forming any further rudiments of enamel-organs. The third 

 premolar is therefore at this young stage far the most deve- 

 loped of all dental rudiments, and already exhibits the earliest 

 indication of the enamel-organ of the successional tootli, while 

 the rest are scarcely difterentiated from the dental fold. The 

 conditions in the lower iaw are precisely similar, though the 

 rudiments are still less developed. 



The next embryo selected for examination was considerably 

 larger, measuring 2'5 centim. Here we tind the development 

 of tlie teeth greatly advanced. Commencing with the incisors, 

 we see how a strong cap of dentine is ditl'erentiated by the 

 odontoblasts. In the enamel-organ the enamel-pulp has 

 become almost completely obliterated owing to the vigorous 

 growth of the dental papilla. The internal enamel-epithelium 

 consists of very columnar and narrow cells regularly disposed 

 side by side ; the external one forms a not altogether thin 

 layer of flattened cells above it. The continuity of the 

 enamel-organ with the epithelium of the cavity of the mouth 

 is still preserved ; at the same time, however, we also notice 

 how, in the case of each of the incisors, from the tolerably 

 broad nock a cord of ei)ithclium projects on the inner side of 



