290 Dr. W. Kiikenthal 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- 
packed 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 which 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 off can only be 
regarded as the earliest rudiment of the enamel-organ of the 
successional tooth. 
Shortly after this the dental fold comes 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 tooth, while 
the rest are scarcely differentiated from the dental fold. The 
conditions in the lower jaw 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 find the development 
of the teeth greatly advanced. Commencing with the incisors, 
we see how a strong cap of dentine is differentiated by the 
odontoblasts. In the enamel-organ the enamel-pulp has 
become alinost completely obliterated owing to the vigorous 
growth of the dental papilla. The internal enamel-epithelium 
consists oi 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 neck a cord of epithelium projects on the inner side of 
