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62 EMBRYOLOGY OF TlIK DENTAL TISSUES.

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Section of jaw, embryo of pig, show in^ '1(\ rldpmcnt (if temporary molar tooth : 1, enamel organ ;
:J, dentin germ.

The Enamel Organ.
The enamel organ is now in its perfected state. On examination
it is found to be composed of three distinct celhilar forms. The essen-
tial layer is the ameloblastic layer of columnar cells which rests upon
the dentin germ. These are the cells that are to become the enamel
cells or ameloblasts. They have become changed by pressure into very
symmetrical hexagons, four or five times as long as they are broad, with
a distinctly marked nucleus in the part farthest away from the dentin
germ. Only the sides of the cells are said to have membranes : they
are without covering at either end. These cells are longer just over the
point of the dentin germ and are shorter as they approach its base, being
here very mucli like those of the outer layer, the external epithelium of
the enamel organ.
This outer layer is composed of cells which are roundish, a little
longer than they are wide, and seem to be losing their columnar form.
Indeed, soon after calcification has commenced these cells disappear.^
^ It is a question what becomes of them. Some authorities think that they are the
origin of Nasmyth's membrane, but this is very doubtful, for investigation shows that