292 Dr. W. Kiikenthal on the 
3 and 3:2 centim. from rump to nape. The epithelial cords 
and their knobbed swellings are traceable with the utmost 
distinctness on the inside of the dental rudiments (vide 
figs. 2-7). 
We now come to the interpretation of the facts observed. 
In all three larger stages we see the dental rudiments dis- 
tinctly developed, and moreover an epithelial cord running 
close to and on the inside of them, which arises from the neck 
of the enamel-organ and is provided with a swollen free end. 
In these lateral cords of epithelium we have before us 
PERFECTLY TYPICAL RUDIMENTS OF THE EARLIEST STAGES OF 
THE ENAMEL-ORGANS OF SUCCESSIONAL TEETH, and they are 
indicated with special distinctness in the case of the whole of 
the incisors; remains of these rudiments are also seen in the 
case of the canine as well as in that of the first molar, and it 
was only in the case of the second premolar that I did not 
succeed in discovering them. A valuable subject for com- 
parison is furnished by the third premolar, to which a succes- 
sional tooth actually appears later on. Now the rudiment of 
the enamel-organ of its successional tooth agrees so entirely 
with that of the rudiments of the other successional teeth that 
there is nothing to prevent their homologization. ‘The dis- 
covery of rudiments of successional teeth in the case of teeth 
other than the third premolar, which Thomas himself declared 
would be fatal to his hypothesis, has therefore been achieved, 
and moreover not in the case of one, but in that of almost all 
teeth. It is for the present a matter of indifference to us 
whether these enamel-organs of the rudiments of successional 
teeth undergo still further development or become rudimentary 
at an early period; in any case the nature of the dentition of 
Didelphys, and, as 1 shall immediately add, in all probability 
that also of the rest of the Marsupials, is settled. THE 
PERMANENT DENTITION OF THE MARSUPIALS BELONGS TO THE 
FIRST SERIES, THE MILK-DENTITION; RUDIMENTS OF THE 
SECOND DENTITION ARE ACTUALLY PRESENT IN AN EMBRYONIC 
CONDITION, BUT WITH THE EXCEPTION OF THE THIRD PRE- 
MOLAR IT DOES NOT CUT THE GUM. 
A few words yet remain to be added as to the rudiments 
of the so-called true molars, the molars. The smallest stage, 
1 centim. in length, showed as yet no trace of a rudiment 
either in the upper or in the lower jaw. It was only in the 
three subsequent larger stages that rudiments of these teeth 
could be detected, and here they were nearly equally far 
advanced in development. In the upper jaw the first and 
second molars were present, in the lower jaw the first, second, 
andthird. In all cases development was already far advanced ; 
