Dentition of Didelphys. 293 
the separate cusps of dentine were well developed, while the 
spaces between the several cusps of the tooth were filled with 
enamel-pulp, which was surrounded by a very columnar 
internal and a flat external enamel-epithelium. 
As has already been mentioned, a cord of epithelium 
running in a lateral direction is also present in the case of 
the first molar. The cord is, however, very little developed ; 
it runs continuously backw ards, and at the level of the dental 
rudiment of the second molar it comes into connexion with 
the enamel-organ of the latter; but simultaneously it sends 
off inwards a second strong cord of epitheiium of considerable 
length, which terminates with a knobbed swelling (vide 
fig. 8). The swelling is surrounded by close-packed strands 
of connective tissue, ‘arranged concentrically, and exhibits at 
its free end two slight indentations, while at the same time 
in the connective tissue, which lies beneath, the first beginnings 
of a papilla become visible. The structure does not lie as it 
might be behind the large rudiment of the second molar, but 
to one side of it, placed at a considerable distance towards the 
interior; and I can therefore not regard it otherwise than as 
the rudiment of a successional tooth. ‘Thus it is demon- 
strated that the second molar (and naturally the first also) is 
in its origin in no way different from the teeth lying in front 
of it. THE TWO FIRST SO-CALLED MOLARS OF THE UPPER 
JAW BELONG TO THE FIRST DENTITION. 
Shortly after the successional tooth has disappeared from 
the scene the second molar also disappears further backwards, 
and no indication appears of the rudiment of the last two 
molars. It is reserved for further investigations, prosecuted 
upon more comprehensive material, to display the earliest 
rudiments of these. 
In the lower jaw the development of the molars has 
advanced further ; the third molar also is already developed, 
somewhat smaller, it is true, than the preceding one, but still 
already provided with all its cusps. Here the conditions are 
such that, from the first molar onwards, an epithelial cord 
runs uninterruptedly through the posterior portion of the 
lower jaw, and, flattened out like a plate, passes above and 
laterally on the inside of the dental rudiments. It soon 
comes to pass that this cord has no longer any connexion 
with the epithelium of the cavity of the “nouth, since the 
latter withdraws more towards the middle in consequence of 
the growing together of the margins of the upper and lower 
jaws. At each enamel- -organ of the three molars a branch 
now passes off from this broad and very conspicuous epithelial 
cord, so that in each case the appearance of a dichotomic 
