160 THE STRUCTURE OF MAN 
in Man, and Rose has since proved that this vestige is on each 
side coincident with the end of the epithelial dental ridge. 
By milk teeth are usually understood the first formed 
generation of teeth. Rose, however, has recently attempted to 
show that the milk teeth do not correspond with the first series 
of teeth of the lower Vertebrates, and that they cannot be 
homologised with any one special series in Reptiles and allied 
forms. Milk teeth, according to him, must rather be considered 
to have arisen by the concrescence of several consecutive 
generations of teeth of our ancestors, into one single, more solidly 
constructed, series, the sum of all the remaining rows which were 
once present having been in Man, as in all diphyodont Mam- 
mals, compressed into the second or permanent series. [This 
is, however, but one of several views put forward during recent 
years on the subject of the Mammalian tooth genesis. Much 
more important is the.fact that, in Man, while the premolars 
are comparatively simple teeth, the milk molars which precede 
them are more complex, and more conformable, in the characters 
of their fangs and crowns, to the type of the true molars. 
These facts suggest that the deciduous (milk) molars are of a 
more primitive (7.e. a less reduced) type than the successional.'] 
Until quite recently, the possibility of Man’s developing a 
third dentition was generally denied, but it 1s now proved that 
that may sometimes occur. Baume, Zuckerkandl, and Rose, have 
discovered a third set of enamelless tooth rudiments on the outer 
or labial surface of the jaw, [and Schwalbe has lately suggested ° 
that they may be the vestiges of a distinct pre-milk dentition, 
of which traces have been found by Kikenthal in the Seal, by 
Nawroth in the Pig, and, in a more extensive and calcified form, 
by Leche in the Banded Ant-Eater (Myrmecobius). Great 
interest attaches to further inquiry into these structures. | 
In Fishes, Amphibians, and some Reptiles, the first formed 
1 [A very interesting allied case is furnished by the common Dog. In the upper 
jaw of that animal, the characters of the fourth milk (deciduous) molar are almost 
exactly those of the first true molar, and the characters of the third milk molar those 
of the fourth premolar. Similarly, the second and first milk molars closely resemble 
the third and second premolars, allowance being in all cases made for mere differ- 
ence in size. Indeed, comparison of the premolars with the milk molars and, through 
these, with the first molar, reveals a marvellous series of progressive stages in 
simplification and reduction of the type of tooth represented in the adult dentition 
by the first upper molar. Iam hoping shortly to have this most important matter 
fully worked out in detail.—G. B. H.] 
2 [Cf. Schwalbe, Morph. Arbeiten, Bd. iii. p. 531, and Nawroth, ‘‘ Zur Ontogenese 
d. Scheweinemolaren,” Znaug. Dissert. Basel, Berlin, 1893. ] 
