Franklin P. Mall 445 
as long as the distance between the maxillary and temporal processes and 
gives the appearance of the horns upon an ox’s head. At no time is a 
second center visible. 
Temporal bone.—This bone also appears on the 56th day as a small 
hook-like nucleus about a millimeter long representing mostly the zygo- 
matic process. The slightly enlarged dorsal end marks the beginning of 
the squamous portion. The primary and only nucleus of the squamo- 
zygomatic gradually enlarges, the zygomatic process growing longer and 
the squamous portion spreading out over the temporal region of the head. 
By the 58th day the squamous portion measures 2.5 mm. in diameter and 
the zygomatic portion is also as long. At no time are these two parts of 
the bone separated, but they are firmly attached to each other as may be 
demonstrated by pressing them between two needles. They gradually en- 
large and on the 65th day a small nucleus appears below the junction of 
the zygomatic process with the squamous portion to which is attached 
the delicate tail-like ring. The tympanic ring is present only on the right 
side of this embryo (282). Gradually the ring enlarges and finally makes 
the circle complete on the 85th day. The squamous portion at first be- 
gins to radiate from its point of junction with the zygomatic, but as the 
bone grows larger an axis is extended partly through the middle of the 
squama from which this net-work of bone now radiates. During this 
time the squamous portion grows below the zygomatic; on the 73d day 
it measures 3 mm. in length, on the 85th day 7 mm., and on the 105th 
day 11 mm. 
Frontal bone——The frontal also appears on the 56th day, a little later 
than the time given by Toldt. In embryo No. 274 it forms a reticular 
nucleus about 4 mm. in diameter with the orbital plate a little more de- 
veloped than the rest of the bone. On the 58th day it measures 8 mm. in 
_ diameter; on the 73d day 10 mm. and on the 85th day 15 mm. 
Parietal bone.—This bone is a little behind the frontal in its appear- 
ance, judging by its transparency and extent. On the 56th day it appears 
as a very delicate reticular nucleus, about 3 mm. in diameter, which can 
be seen only with difficulty. A few days later (272) it is found spreading 
towards the occipital bone and the middle line. It is now hour-glass 
shaped, each end of which is about 4 mm. in diameter and may represent 
the two centers described by Toldt. At this time the nucleus near the 
sphenoidal angle is more extensively ossified, and its reticular structure is 
coarser than in the nucleus near the occipital angle of the parietal bone. 
The bone now grows rapidly, keeping pace pretty well with the frontal. 
On the 105th day it measures 22 x 26 mm. 
