440 Ossification Centers in Human Embryos 
condyle has become much more sharply defined and its bone fibers again 
reach to the symphysis. About this time the mylohyoid line and the 
lingula appear. On the.83d day the ramus is becoming relatively thinner 
and broader, the coronoid process has moved farther away from the con- 
dyle, the angle has become more marked, and the alveolar process has in- 
creased in length. The mandibula has now its characteristic shape, and 
measures 14 mm. in length. In older embryos its form is characteristic ; 
it gradually increases in size, measuring 19 mm. in embryo P. 
Mazilla—The maxilla, according to my specimens, arises from two 
centers only, one to form the premaxillary bone and one to form the main 
body. In one of the youngest specimens (263, b) the maxilla is marked 
by a mass of granules, together one-half millimeter in diameter, lying 
spread out just beneath the eye and one millimeter from the middle line. 
In another specimen (C) it is impossible to find the maxilla, but a very 
small premaxillary is found measuring one-fourth of a millimeter in 
diameter. A second specimen of the 42d day (42) shows both centers 
present as granular masses. The maxilla is just beneath the eye, over a 
millimeter in diameter and the premaxillary is as small as in C and sep- 
arated by a millimeter from the maxilla. A little later (56 and 202) the 
two bones are denser in structure and both have parallel processes which 
no doubt are to form the frontal process. These two bones are found 
united along the alveolar border on the 56th day (274), but the frontal 
processes remain separated for a long time. In another embryo of the 
56th day (263, b, 2) the frontal process is found to be distinctly double, 
one-half coming from each center, and the alveolar process is large and 
includes both of them. The orbital plate and the palatine process are just 
beginning and the zygomatic process is well developed. By grasping the 
different sides of the bone between two needles it is easily demonstrated 
that there is but one bone from this time onward. If much pressure is 
exerted it is observed that the bone bends, and if too much, it breaks. 
In later stages when the palate, temporal and zygomatic bones are present 
it is easily seen that these are separate, although they come in contact with 
the maxilla. At no time are more than two centers present, and these 
unite in the very beginning of the third month. In Table II the parallel 
lines are inserted between the columns for the maxilla and the premaxil- 
lary for specimens, in which these two bones can still be recognized, but 
they are firmly united. However, these processes, especially the zyogmatic, 
are easily broken off, and, judging by the illustrations of this bone in 
Rambaud & Renault, their numerous centers are due to such breaks. In 
the next specimen (266) the premaxillary is separate, and not united 
with the body of the maxilla; in all of the older embryos they are united. 
