34 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
ment be spoken of. In certain cases the whole depth of the enamel 
ridge passes into the enamel germ, and at such a place there is usually 
no succession. It seems that the continuauce of a connection between 
the buccal epithelium and the enamel ridge implies a continued pro- 
ductivity in the latter. The replacement teeth are not derivatives of 
the milk-teeth, but both arise (the younger to the lingual side) from the 
common ridge. 
A dentition is a generation of teeth, and Leche supports the view 
that the second set was probably first acquired by the placental Mammals* 
The serial succession in time and space was a gradual result following 
on the differentiation of individual components. Reduction follows 
either from intense division of labour and specialisation of teeth, or from 
change of diet. On the whole, monophyodontism in mammals points to 
the suppression of the first dentition. Though the arguments are not 
conclusive, the probabilities are that the molars belong to the first 
dentition. 
There may be representatives of a third dentition (hedgehog, seal,, 
man), an interesting case of the acquisition of new parts. But there are 
also hints of a pre-milk dentition (hedgehog, opossum, &c.). Leche 
gives evidence in favour of the position that there may be an increase in 
the number of teeth in Mammals, while most believe only in the possi- 
bility of decrease, and he ends by summing up the arguments against 
the theory of coalescence — the origin of multituberculate teeth from the 
fusion of simple cones. 
Questions concerning Dentition.* — Prof. W. Leche agrees with 
Klikentbal in regarding the ante-molar teeth of Placentals, except 
pm. 3, as persistent milk-teeth, the replacement-teeth being represented 
merely by enamel germs lying to the tongue side of the persisting teeth* 
But he has two questions to ask : (1) Why does pm. 3 alone develop as 
a replacement-tooth ? and (2) Are the enamel germs in a progressive or 
in a retrogressive state ? Leche supposes, in answer to the first question, 
that the development of a suctorial mouth, characteristic of all young 
Marsupials, has inhibited the second dentition. He differs from Kiikentkal 
in regarding the enamel germs as incipient rather than vestigial : they 
last longer than the enamel ridge ; it is difficult to see why a second 
set should have been suppressed rather than the first ; in Erinaceus and 
Phoca teeth sometimes develop from similar bud-like swellings to the 
lingual side of the persisting teeth ; in the Jurassic Triconodon serrula 
only the last premolar has a successor, just as in modern Marsupials. 
Leche goes on to criticise Kukenthal’s interpretation of the buds as. 
vestiges. 
Nor does he agree with the view that the teeth of Odontoceti are 
milk-teeth, noting that in other cases of monojdiyodont dentition the 
first set disappear, and that Zeuglodon had a typical succession of teeth. 
But he leaves the question open. The contrast between Leche’s views 
and Kukenthal’s comes in general terms to this, that the latter allows 
only retrogressive reduction in the number of dentitions in Mammals, 
while the former sees evidence of progressive increase as well. 
Development of Teeth in Insectivora.j* — Mr. M. F. Woodward, 
from a study of the teeth of various Insectivores, comes to the conclu- 
* Anat. Anzeig., xi. (1895) pp. 270-6. f Rep. Brit. Ass., 1895, p. 736. 
