Franklin P. Mall 439 
method of preservation and the amount of distortion, but neither of these 
factors influences the order of ossification. I may add that the length 
of the embryo was determined by direct measurement from crown to 
rump, but in case the embryo was distorted this measurement was de- 
duced from the neck-rump length which is the least variable of external 
measurements. The age of the embryos in days was obtained by multi- 
plying the square root of the crown-rump length in millimeters by ten.” 
These ages correspond with the estimations given by embryologists. If 
in the course of time our data enable us to determine the ages more “ac- 
curately, the ages I have ascribed to the various embryos can easily be 
changed, for in all the specimens the length from crown to rump is also 
given. 
OSSIFICATION CENTERS IN THE SKULL. 
Mandibula.—The mandible appears a very little later than the clavicle. 
In two embryos, 15 mm. long, 39 days old, it is present as a finely granular 
or reticular mass, half a millimeter long, immediately below the epidermis 
towards the free end of the first arch, representing, of course, the body of 
the jaw. In the next specimen, 263, b, the mandibula is found to be a 
slender but compact bone about one millimeter long reaching nearly to 
the midyentral line; in D it is a little larger and more sharply defined. 
On the 42d day (No. 42) the bone measures 2 mm. and shows the begin- 
ning of the ramus and of the alveolar process. It now gradually en- 
larges, measuring 3 mm. in No. 56 and No. 333. The lower 
jaw in these embryos is thin and transparent, showing a delicate struc- 
ture. The alveolar process and the ramus are very transparent, while 
the base and the part of the body near the midventral line is thick and 
opaque. Much the same appearance is seen in No. 202 (55 days), while 
in 274 the jaw is fully 5 mm. long and shows the beginning of the coro- 
noid process and the condyle. A day later, No. 263, b, 2, the jaw is 
6 mm. long, and within it can be seen three lines of thickened bone radi- 
ating from the symphysis, one towards the angle, one into the condyle, 
and one into the coronoid process. The next day a few sockets for the 
teeth may be seen in the alveolar process. This condition remains, the 
jaw only growing in size, until the 58th day (272) when the jaw appears 
hollow and the three radiating lines no longer reach to its anterior end. 
By the 75th day (288, b) the jaw has grown to be 10 mm. long and 
meets its: fellow to form the symphysis on the midventral line. The 
See Mall, Amer. Jour. of Anat., II, 335; Johns Hopkins Hospital Bulletin, 
1903; and Johns Hopkins Hospital Reports, IX, 1900. 
