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GENERAL ACCOUNT OF EMBRYONIC DEVELOPMENT. 553

part of the bar constitutes IMeckel's cartilage (ink). In the course of
further development the Meckelian part of the mandibular arch becomes
enveloped in a superficial ossification forming the dentarv. Its upper
end, adjoining the quadrate region, becomes calcified, and then absorbed,
and its lower, with the exception of the extreme, is ossified and subse-
quently incorporated in the dentarv." Tomes says : " About the fortieth
day a centre of ossification appears in the mandibular process, whicli,
spreading rapidly, soon forms a slight osseous jaw outside Meckel's car-
tilage, which is not, however, in any way implicated in it, and very soon
begins to waste away ; so that by the end of the sixth month it has dis-
appeared. That end of it alone which extends up to the tympanum
does not waste away, but becomes ossified into the malleus. There are,
however, observers who hold that in some animals, at all events, Meckel's
cartilage plays a more active part in the ossification of the jaws." Dean
agrees with the authors above quoted on all essential points.
The disappearance of Meckel's cartilage is accomplished by calcifica-
tion, and afterward by ossification. The development of bone in the
jaw begins in the embryonic connective tissue surrounding Meckel's
cartilage ; the latter, occupymg the central portion of the jaw, is sur-

2210.

Meckel's Cartilage, from jaw of t-wo-and-a-half months' human foetus undergoing ossification : n, nor-
<; enlarged cells containing calcitic material ; db,db, developing bone cl, con-
mal cartilage-cells; ;
nective tissue (X '2r)0).
rounded by the connective tissue of the mesoblastic layer, which in turn
is covered externally by that of the epiblast.
Development of bone in the inferior maxilla begins in the mesoblastic
layer prior to the differentiation of the periosteum, about the fortieth
day. As the forming bands of bone thicken they encroach upon
Meckel's cartilage, which is also undergoing calcification. Under the