54.4, Je TL. WILSON AND J. P. HILL. 
Kiikenthal, who laid special emphasis upon the precocious 
differentiation of dp.8 in Didelphys, held that the early 
liberation of the residual dental lamina entailed by the con- 
striction off of the enamel-organ of dp.3 could only be con- 
ceived as the differentiation of the first Anlage of the succes- 
sional tooth (7). It will be plain from the views we have 
enunciated throughout this paper that we cannot assent to 
this interpretation. The residual lamina beside dp. 31s not to 
be conceived as actually at its first appearance a rudimentary 
enamel-organ. It is at first indifferent. But it is not long 
before a definite and progressive localised thickening and en- 
largement sets in (cf. Stage 111) similar to that which constitutes 
the earliest rudiments, say, of the other premolars. With the 
latter, indeed, the rudiment of p. 3 is in direct serial continuity. 
We may therefore very well consider the rudiment of p.3 as 
appearing tolerably early in development, though its further 
structural evolution is greatly delayed by the development 
beside it of its predecessor dp. 3. 
II. Interpretation of the Tooth-change in 
Marsupials. 
In reference to the second question formulated, it will be 
borne in mind that all recent authors are agreed that in those 
Marsupials whose dentition has been satisfactorily investigated, 
one tooth only is replaced by a successor, and that this tooth 
is invariably the last premolar. 
The statement of Owen (15) that in Macropus, and also in 
wombat, the milk incisors are shed during the mammary 
foetal stage, has never been confirmed by any subsequent 
observer, and has been universally discredited, the more readily 
that the statement is made without any definite indication of 
the details of such a process having been actually observed. 
It is, nevertheless, hard to believe, as Woodward (14) has 
pointed out, that such an accurate observer should hazard a 
statement of this kind absolutely without evidence. And in 
view of the discoveries of Woodward and Rise regarding the 
presence of vestigial teeth in the forms named, it may yet be 
