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TEETH OF THE VERTEBRATA. 361

somewhat of the nature of mucous membrane proper. The real point
of blending is, in the embryo at least, not at the lips, but lies inside the
borders of the jaws. If, therefore, we limit the term " mucous mem-
brane" in this situation to that tissue which is of hypoblastic origin,
then the teeth of the jaws cannot be said to be developed from the
mucous membrane of the mouth, as is commonly stated, but from the
invao-inated inte(>;ument.
In many fishes teeth are found far back in the pharynx, and are
attached to the gill-arches and pharyngeal bones. I am informed by
Mr. J. A. Ryder, whose extensive knowledge of the embryology of
fishes renders his statements highly authoritative, that these teeth lie
beyond the limits of the invaginated integument, and are truly of hypo-
blastic derivation. If this be true, the generalization that all teeth are
modified dermal spines is certainly incorrect. It affords us, however,
an example in which identical structures have been produced from tissue
of vastly different origin in a similar manner, and in all probability
attrii^utable to the same causes—viz. repeated stimulation of a paiticu-
lar point, which eventually gave rise to a calcified papilla.
The point at which a tooth is about to be developed is marked by a
proliferation of the cellular elements of the tissue in which it will ulti-
mately appear. These eventually arrange themselves into three organs,
which have been denominated the dentine organ, the enamel organ, and
the dental sacculus. This latter organ becomes so modified in some ani-
mals, in which coronal cement is extensively developed, as to merit the
distinction of cementum organ. Taken collectively, tliey represent the
tooth-germ. C. S. Tomes very justly remarks that "the tooth is not
secreted or excreted by the tooth-germ, but an actual metamorphosis of
the latter takes place." The three principal tissues, dentine, enamel,
and cementum, thus produced, are formed from their respective organs,
and consequently separate parts of the tooth-germ. Although many
adult teeth do not possess enamel upon their crowns {e.g. edentates or
sloths, armadillos, etc.), yet the presence of an enamel organ in the early
stages of growth is believed to be a universal feature of the development
of all teeth, and is one of the strongest arguments for their community
of origin, however much they may have been subsequently modified.
The Enamel and Dentine Organs.—In the earliest stages of the
development of a mammalian tooth, which is here taken for descrip-
tion, a slight longitudinal depression in the epithelium covering the bor-
ders of the jaws is noticeable ; this is somewhat augmented in depth by
the addition of a ridge upon either side of it. At the bottom of this
groove the deepest or Malpighian layer of the epithelium grows down
into the corium as a continuous fold or lamina, being directed down-
ward and a little inward. In cross-section this fold resembles a tubu-
lar gland and extends throughout the entire length of the jaw. In the
positions where teeth are to be formed the lower extremity of this
lamina is considerably enlarged by the rapid multiplication of its con-
stituent cells. The continuity of the fold is now broken up, and the
structure which is destined to become the enamel organ appears as a pro-
cess of epithelium comparable in shape to a Florence flask (Fig. 189).
The outermost layer of the organ at this stage is made up of cells of