Embryology. 549 
forms the enamel of the crowns of the molars develop hard enamel, 
but the portion which extends down over the root and cervix of the 
molars undergoes degeneration, and its cells lose their columnar 
form and degenerate into radiating fibres of considerable length, 
which send their free ends into the surrounding alveolar perios- 
teum. These fibres persist even to adult age, and can be readily 
seen extending from the tooth into the wall of the alveolus in sections 
of the entire heads of adult white mice (Mus musculus) prepared by 
the present writer during the last winter. These fibres evidently serve 
to securely anchor the teeth into the alveolus of the adult, so that 
the enamel organ is found to have not simply the function usually 
ascribed to it, but another equally important, namely, the produc- 
tion of these anchorage fibres. Still deeper down in the alveolus 
the extreme inferior edge of the cap or dome-like enamel organ 
Seems to become quite degenerate and functionless. Such function- 
less marginal portions of the enamel organ are found in the young 
of man, the ox, and the rat and mouse. The enamel organ is re- 
garded by Von Brunn in fact as a sort of mould in which the den- 
tine or pulp covered with odontoblasts assumes a definite form. 
Von Brunn concludes his second memoir with the observation that 
he considers that he has shown that in the Mammalia, wherever 
dentine is developed, that the epithelial sheath or cap concerned ' 
in the formation of enamel must have first existed. This explains 
the existence of the enamel organ in the armadillo (Tomes, Q. 
Jour. Mic. Sci., 1874). 
The enamel-forming portion of the enamel organ, after eruption 
of the molars, is, of course, cast off entirely, or at most persis 
only as the enticula dentis. The portion giving rise to the an- 
chorage fibres of the root persists, as may be seen in longitudinal 
sections of the molar teeth of adult animals in place in the jaw. 
the incisors of the rat, on the contrary, the enamel organ has a 
more complex history. While it does not differ entirely from the 
germs of the other teeth in an early stage of development, as all 
parts of its wall are alike thick, later the anterior wall of this 
primary enamel organ becomes the persistent enamel organ of the 
enamel band on the anterior face of the incisors, and thickens, 
While over the sides and back of the tooth it degenerates and gives 
rise to the anchorage fibres of these teeth, as shown by Von Brunn, 
and as confirmed by sections of later stages in my possession 
enamel organs of the first and second molars are connect y an 
isthmus, and it is probable that the germs of the second and third 
molars are also thus joined together. In longitudinal sections of 
the heads of young mice it is, however, of great interest to note 
that the cross crests of the upper molars have their apices directed 
