Page 343 - My FlipBook
P. 343
"
TEETH OF THE VERTEBRA TA. 353
teeth of the lampreys among vertebrates. If the term " tooth
is applicable to these structures, then we must likewise include the
" baleen " of the Cetacea and the beaks of birds and reptiles, which by
common consent are far removed from true teeth. For all such I think
the term oral armature is preferable, from the fact that their produc-
tion not infrequently depends upon the modification of organs widely
different in origin.
On the other hand, those of enderonic source are found only within
the limits of the Vertebrata, and range in form from the simple cone
usual among fishes to the higher complex grinding organs of certain
herbivorous mammals. They all agree in being developed from the
corium of the lining membrane of the mouth, which is continuous with,
and really a part of, the integument, invaginated at an early period.
There is a possible exception in the pharyngeal teeth of fishes, which
Ryder considers to be of liypoblastic origin or developed from the base-
ment-layer of the mucous membrane of the alimentary canal, and which
are practically the same as those of epiblastic origin, as far as their
relation to the surface is concerned.
When we speak of teeth being modified dermal appendages, it will
not be amiss to cite the evidence upon which such a generalization rests.
Tliis is best afforded by a study of the relationship and development of
the dermal armature of certain elasmobranch fishes, of which the shark
is a good example and furnishes us with one of the earliest, and there-
fore one of the most primitive, conditions of the Vertebrata.
In these fishes the defensive power of the integument is augmented
by the production of numerous hard bodies in its substance, which have
been termed " dermal denticles " by Gegenbaur. These structures,
which are likewise known as " placoid scales," are distributed over
the whole of the integument in shark-like fishes, and are ordinarily
Fig. 187.
Vertical Section through the Sliin of an Embryonic Shark: r, corium ; c, p, c, layers of corium ; d,
uppermost layer /*, papilla; ^, epidejmis ; e, its layer of columnar cells; o, enamel layer (from
;
Gegeubaur, after Uertwig).
rhomboidal in form, with their apices directed obliquely backward.
They consist of a solid body, which is inserted by its base into the
Vol. I.—23
TEETH OF THE VERTEBRA TA. 353
teeth of the lampreys among vertebrates. If the term " tooth
is applicable to these structures, then we must likewise include the
" baleen " of the Cetacea and the beaks of birds and reptiles, which by
common consent are far removed from true teeth. For all such I think
the term oral armature is preferable, from the fact that their produc-
tion not infrequently depends upon the modification of organs widely
different in origin.
On the other hand, those of enderonic source are found only within
the limits of the Vertebrata, and range in form from the simple cone
usual among fishes to the higher complex grinding organs of certain
herbivorous mammals. They all agree in being developed from the
corium of the lining membrane of the mouth, which is continuous with,
and really a part of, the integument, invaginated at an early period.
There is a possible exception in the pharyngeal teeth of fishes, which
Ryder considers to be of liypoblastic origin or developed from the base-
ment-layer of the mucous membrane of the alimentary canal, and which
are practically the same as those of epiblastic origin, as far as their
relation to the surface is concerned.
When we speak of teeth being modified dermal appendages, it will
not be amiss to cite the evidence upon which such a generalization rests.
Tliis is best afforded by a study of the relationship and development of
the dermal armature of certain elasmobranch fishes, of which the shark
is a good example and furnishes us with one of the earliest, and there-
fore one of the most primitive, conditions of the Vertebrata.
In these fishes the defensive power of the integument is augmented
by the production of numerous hard bodies in its substance, which have
been termed " dermal denticles " by Gegenbaur. These structures,
which are likewise known as " placoid scales," are distributed over
the whole of the integument in shark-like fishes, and are ordinarily
Fig. 187.
Vertical Section through the Sliin of an Embryonic Shark: r, corium ; c, p, c, layers of corium ; d,
uppermost layer /*, papilla; ^, epidejmis ; e, its layer of columnar cells; o, enamel layer (from
;
Gegeubaur, after Uertwig).
rhomboidal in form, with their apices directed obliquely backward.
They consist of a solid body, which is inserted by its base into the
Vol. I.—23
