column develops as a segmented structure
round the notochord; the skull develops first as an unsegmented plate
extending far beyond the notochord. The processes of this basilar plate,
the trabeculae, are quite unlike anything in the vertebral column. It is
true that when the process of ossification begins, separate bones are
differentiated in the basilar plate one in front of the other, giving an
appearance of segmentation. The hindmost of these bones, the
basioccipital, ossifies round the notochord, quite like a vertebral
centrum, and its side parts which form the occipital arch develop in a
"remotely similar" way to the neural arches of the vertebrae. The next
bone, however, the basisphenoid, develops in front of the notochord, and
shows very little analogy with a vertebral body. The analogy is even
more far-fetched when applied to the axial bones in front of the
basisphenoid. The cranium might indeed be divided upon ossification into
a series of segments bearing a more or less remote analogy with
vertebrae. "In the process of ossification there is a certain analogy
between the spinal column and the cranium, but that analogy becomes
weaker and weaker as we proceed towards the anterior end of the skull"
(p. 585). The best way to state the facts is to say that both skull and
vertebral column start in their development from the same point, but
immediately begin to diverge. The clear indications of segmentation
which fully ossified adult skulls undoubtedly show are, therefore,
secondary, and the vertebral theory of the skull, which was originally
based upon the appearance of such fully ossified crania, is on the whole
negatived by embryology.

We have now to turn back a few years in order to follow up another line
of discovery which had an important bearing upon the theory of the
vertebrate skull--the working out of the distinction between membrane
and cartilage bones.

As early as 1731, R. Nesbitt,[222] in two lectures delivered to the Royal
College of Surgeons, demonstrated that in the human foetus some bones
were formed not in cartilage but directly in fibrous tissue, and this
observation was confirmed by other human anatomists, particularly by
Sharpey at a considerably later date. In 1822 Arendt[223] focussed
attention upon the remarkable structure of the skull of the Pike, with
its cartilaginous brain-box studded all over with bony plaques, an
arrangement which had already attracted the interest of Cuvier and
Meck