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Prof. Macalister. Variations in the ossification 
Variations in the ossification of the Occipital hone. By Prof. 
A. Macalister, President. 
\Read 4 March 1901.] 
In one occipital bone out of every four some recognisable 
divergence from the described normal type is to be found specially 
in the region of the planum interparietale. These are either 
varieties in degrees of curvature, ridging or other qualities of 
surface, or varieties in the union of the component elements. At 
present I propose to deal only with the latter series. 
The portion of the occipital behind the foramen magnum 
originates partly in cartilage, partly in membrane. The former 
portion, the supraoccipital, includes all the planum nuchale as 
far as the linea nuchge suprema, the latter includes all that part 
of the planum interparietale which extends from the linea nuchas 
suprema to the lambdoid suture. The supraoccipital region 
ossifies from a pair of centres which arises about the ninth week 
close to the region of the protuberance. These unite medially 
about the twelfth week. Apparently there is not much variation 
in these except in the extent to which they spread. I have 
already described the occurrence of intercalary centres between 
the exoccipital and the supraoccipital, and the os Kerckringii has 
been long known. When it does not exist a median spur derived 
from the united supraoccipital centres can be usually found in 
the foetal skull which enters and, by expanding and uniting 
with the adjoining exoccipital borders, fills up the infra- occipital 
fontanel. 
It is the region of the planum interparietale which is most 
prolific of varieties. My observations lead me to believe that 
normally there are at least four spots at which ossification begins 
in this membranous area. These begin about the twelfth week 
most commonly as a little cluster above the region of the pro- 
tuberance which generally unite on the median part of the space 
about the fourteenth week. As the position of these clusters 
of bony nodular reticulations is inconstant, there is sometimes 
a difficulty in their analysis, and often they appear to be grouped 
from the first as a pair of almost confluent nuclei. 
