of the Skull and the Skeleton. 355 
the epiphysis takes its origin. But if it were possible that the 
tension on the neural arches were ever sufficient to produce an 
impact, then we might reasonably expect that the neural arch 
itself, like the centrum, should have epiphyses, as, indeed, appears 
sometimes to be the case between the zygapophyses. And in 
fishes, where the head is very large and the connexion with the 
body powerful, there appears sometimes to be such an epiphysis 
developed, though it is, as perhaps was to be expected, rather an 
epiphysis of the skull than of the atlas. Thus we are told, by 
Mr. Robertson and others, that in the carp, for instance, if the 
bar of bone which bounds the posterior extremity of the exocci- 
pitals be traced from above downwards, distinct traces of sutures 
will be seen between it and the exoccipitals on which it rests ; 
and following it upwards another suture is found dividing it 
from the supraoccipitals, so that the bars do not meet above to 
form a complete arch, the supraoccipitals being prolonged back 
between these two plates and forming the upper part of this 
neural arch, which has no centrum of its own, but rests on the 
basioccipital. Thus it is seen that epiphyses are not limited to 
the limb-bones and centra of the vertebrae, but that they may 
be developed on any bone if it is subjected to the requisite ten- 
sion and pressure. 
And from these considerations I deduce the following theory 
of the vertebra—viz., that it consists of a centrum or centre of 
ossification which normally developes three (or more) pairs of 
epiphyses, any of which may assume the appearance of separate 
bones and develope epiphyses themselves. Thus in the majo- 
rity of animals there are, 1st, one pair of epiphyses at the front 
and back ends of the centrum ; 2ndly, one pair above, to enclose 
the neural canal; and, 3rdly, another pair to enclose the viscera. 
The upper epiphyses are observed to change their position a 
little with function, while the lower epiphyses may ascend the 
centrum and become articulated to, and seemingly developed 
from, the upper epiphyses; all of them may be absent, and 
the simple original osseous centre will still be accounted a ver- 
tebra. But, as we shall hereafter see that the whole skeleton 
may by this law be accounted for and derived from a single 
ossification, it would be impossible to admit as a vertebra any 
structure which varied in plan and function from that which is 
found in the spinal column. 
With this conception of a vertebra it will now be possible to 
determine what the skull and spinal column have in common, 
and how far they differ. 
Amphioxus lanceolatus appears to demonstrate that in certain 
vertebrata, where the vertebrate structure is scarcely assumed, a 
skull need not exist, and that there may be nothing in structure to 
