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552 DENTAL EMBRYOLOGY AND HISTOLOGY.
In the viviparous family the perforation of the foetal septum occurs,
in most cases, before birth ; when, however, it does not occur, there
arises a pathological condition known as impervious oesophagus.
Of necessity, the lining membrane of the mouth is formed from the
involution of the epiblast. The point of union between the epiblastic
and hypoblastic layers is located at variable distances from the upper
end of the oesophagus, even to the union of the latter with the cardiac
end of the stomach.
Meckel's Cartilage.—The central portion of the inferior maxilla, very
soon after the union of the two lateral processes, becomes differentiated
into a cartilaginous cord which serves to strengthen the jaw. To this
band or cord the name of the
Fig. 298.
discoverer, Meckel, has been
given. It is formed in two
parts arising from the mallei
of either side, which unite, as
do the lateral processes of the
jaw, at the symphysis mentis.
The cartilaginous matrices of
the bones of the ear become
directly ossified, as does the
MeckeVs Cartilage from Human F-mbryo of Forty to Meckelian cartilage of the jaw.
Foity-two days, and before the appeaiance of the
maxilla y bone (mugnitied ten diameters): A, en- The former undergo ossification
largement of cartilage near its union with the neck
of ma leus; D, union with cartiage of opposite side; about the third month.
M, head of malleus ; N. handle of malleus : IC, carti-
lage of tlie incus; i, cartilage of the stapes; O, car- Regarding Meckel's cartilage
tilage 01 the OS lenticulare The outlines of a jaw very little has been written, and
have been added to the figure to show the relative
position occupied by the cartilag in the jaw. (After I deem the reason to lie in the
INIagitot and Kobin,"liean's translation.)
fact that very little study has
been given to the subject of maxillary ossification. Foster and Balfour,
speaking of the development of the mandibular arch of the chick, say:
" In the inferior maxillary process two developments of cartilage take
place—a proximal and a distal. The proximal cartilage is situated at
the side of the periotic capsule, but is not united with it. It is known as
the quadrate, and in the early stage is
merely a small knob of cartilage. The
quadrate cartilage ossifies as the quad-
rate bone, and supplies the permanent
articulation for the lower jaw. The
distal rod is called Meckel's cartilage
;
it soon becomes covered by investing
(membranes) bones, which form the
mandible, and its proximal end ossi-
fies as the articulare.'"
Embryo Pig an inch a"d a third long ; side Regarding its development, Foster
view of MandilMilar and 1 void .Aichcs
1
(Parker): iri, tongue; ink, Meckelian carti- and Balfi)ur simj^ly quote from Par-
lage; ml. body of malleus; mh, manubri-
uni or handle of the malleus; /'//, tegmcii ker's account of the pig: ''In a some-
tympani ; '. incus; .«', stapes The rest of
the letters refer to the hyoid arch. what later stage (Fig. 299) the upper
end of tlie mandilmlar bar, without be-
coming segmented from the ventral jiart, becomes distinctly swollen, and
clearly corresponds to the quadrate region of other types. . The ventral