Dentition of Didelphys. 287 
teeth should be present, internal to rudiments of teeth which 
are subsequently cut, the proof would thus be furnished that 
the series of teeth which arrives at development belongs not, 
as was hitherto generally believed, to the second, but to the 
first dentition. Thus it would be shown that the milk- 
dentition is not to be regarded as a new and secondary acqui- 
sition within the Mammalian class. 
The very fact that the third milk-premolar is cut at about 
the same time as the other premolars, whereupon the molars 
appear, commencing from the first, and that the third premolar 
which replaces it develops much later than the other teeth, 
especially than its two neighbours *, gives ground for the con- 
jecture that the third milk-premolar belongs to the same series 
as the rest of the teeth which are situated in front of it. ‘This 
difficulty of regarding the third milk-premolar and the other 
teeth as belonging to two nae series was felt by Winge, 
who believed he was able to remove it by explaining that the 
other teeth, in spite of belonging to the second dentition, are 
cut simultaneously with the single milk-tooth because their 
precursors are wanting. Perfect clearness is naturally attain- 
able only by means of an embryological investigation, ‘The 
material at my disposal consisted in the first place of a number 
of lower jaws of young stages of Didelphys, for which I am 
indebted to the kindness of Prof. M. Furbringer; my thanks 
are also due to Dr. Kraepelin, the Director of the Natural 
History Museum at Hamburg, who afterwards handed over 
to me for treatment a number of well-preserved young speci- 
mens of Didelphys, through the heads of which series of 
frontal sections were made. The two smallest evubryos 
examined measured | centim. in length from the rump to the 
nape of the neck. 
{ select the upper jaw for the purpose of description, since 
the conditions in it are more distinct than those in the lower. 
Throughout the entire length of the upper jaw there runs a 
cord of epithelium, the dental fold (“‘ Zahnleiste”’), close 
beneath the epithelium of the cavity of the mouth ; in front 
it is not sharply separated from the epithelium of the oral 
cavity, but further back, on the contrary, it lies at a greater 
depth. ‘The rudiments of the enamel: organs of the tive 
incisors appear as knobbed thickenings of the dental fold. 
Nothing is yet to be seen of the invagination of the enamel- 
organ by the dental papilla; no indication whatever of the 
latter is as yet presented by the rudiments of the incisors. 
The connective tissue surrounding the epithelial knob has 
* Jide Thomas, loc, cit. p. 452. 
