274 Proceedings of Royal Society of Edinburgh. [sess. 
Skull. — As the growth and ossification of the skull is a very 
complex process it affords numerous illustrations of the above 
generalisations. Thus the various membrane bones are normal, 
the parts cartilaginous at birth are of their usual size and form, 
while those bones which are ossified from cartilage at an early 
period of foetal life are considerably modified. Further, we find 
at the base of the skull a marked synostosis of certain bony 
centres. After the entire head with the brain in situ had been 
well hardened in Muller’s fluid, a sagittal mesial section was made 
with a large knife. Similar preparations were made of the head of 
a normal nine months’ foetus for purposes of comparison. In the 
sections thus obtained, the principal alterations in the base of the 
skull are well seen. (See Plate II. figs. 1 and 2.) 
The part of the base which extends from the foramen magnum 
to the anterior edge of the pre-sphenoid is abnormally short, 
measuring 2'4 cm. only, as compared with. 3'6 cm. in the normal 
foetus. This shortened portion of the base is represented by a 
single osseous nucleus, the os tribasilare of Virchow, so called 
because it corresponds to the three nuclei — basi-occipital, post- 
sphenoid, and pre-sphenoid, which Virchow found united in 
the skull of a cretin. 
This single osseous nucleus consists of cancellous bone, its 
margins are regular, excepting the anterior which is markedly 
irregular, presenting a number of tapering processes which project 
forwards into the cartilage. This nucleus is usually regarded as 
being formed by the fusion of three originally distinct nuclei 
by a process of synostosis ; the appearance of the anterior 
margin, as above described, suggests the probability that, at any- 
rate, the pre-sphenoid element has been formed by an extension 
from an osseous centre lying posterior to it, and not from a separate 
centre. 
The mesial portion of the base of the skull in front of the 
sphenoid being normally cartilaginous at birth, is in this specimen 
of normal length. Thus the distance from the anterior edge of 
the os tribasilare to the level of the fronto-nasal suture was 3 cm., 
and in the normal foetus the distance was the same from the anterior 
edge of the pre-sphenoid nucleus to the fronto-nasal suture. The 
continuation of the mes-ethmoid cartilage under the nasal 
