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54 EMBRYOLOGY OF THE DENTAL TISSUES.
union occurs at the median line. The central portion of the arch thus
formed, very soon after the union of the two lateral })rocesses, becomes
differentiated into a cartihifjinous cord or band, which serves to strengthen
the embrv(mic jaw. This is Meckel's cartilage. It is formed of two
})arts arising from the mallei of the ears and traversing both sides of the
embvronic jaw to the point of union. While the jaw-bone is forming,
Meckel's cartilage disappears by absorption ; some authorities believe it
becomes ossified, forming part of the inferior maxilla.
The Embryonic Mucous Membrane.

If at a time just previous to tooth formation a section across the
lower jaw is cut, it will be found to consist of a central mass of
embryonic connective - tissue cells
edged on every surface by the in-
nermost layer of the epithelium.
This covering of epithelium is the
Malpighian or mother layer, most
important to the dental histologist,
because from it originate the en-
amel organs of the teeth, as well as
' the bulbs of the hair and the epi-
thelium of the glands. Thus early
the Malpighian layer consists of
cells somewhat like those of the
connective tissue within, but they
Section of jaw, embryo of pig, showing the stain more deeply and are really
appearance of mucous membrane before CClls, having their origin
the formation of the enamel organ : 1, epi- r ^ithclial ' fe &
thelium ; 2, stratum Malpighii ; 3, embry- from the Cclls of the Cpiblast. This
onic connective tissue. tv r i • i • i
Malpighian layer is, again, every-
where covered by epithelial cells, which are continually formed by it.
When the tissue is older, the cells of the stratum Malpighii become
columnar or prismatic in shape, standing somewhat vertically over the
emV)rv()nic tissue beneath. They have large round nuclei, and some
authors have stated that they have no cell-wall. Just without these
are larger cells, sometimes called youthful cells, and external to these
the cells are larger and are more polygonal in form, representing
the cells in their middle life, in which the cell-wall has increased in
thickness, while the nucleus is found to be smaller. Those cells on
the outer surface are the aged cells, consisting almost wholly of formed
material. They in time lose their vitality, having undergone changes,
until, from the fresh mass of protoplasm, they finally become thin, lifeless
scales, which in adult tissue are constantly cast off during the life of the
individual. They are reproduced from the cells of the stratum Malpighii.