360 Mr. H. G. Seeley on a Theory 
surface of that bone, and to enter into the condyle. But they 
differ from the inferior epiphyses of vertebre in being united and 
never surrounding any vessels; and therefore, perhaps, they are 
rather to be regarded as distinct ossifications peculiar to the skull. 
As we have already remarked, the mouth is the prehensile end 
of thedigestive canal, and in Amphioxus it is surrounded by jointed 
rings of cartilage. And, ascending in organization, it were easy 
to trace, by way of the lampreys and sharks, the gradual union 
between the jaws and the skull; and therefore we have to dis- 
cover the origin and the law which governs the uniformity of 
development of these bones of the face. 
And here I seek the aid of embryology to resolve the bones 
into their natural groups, though somewhat reluctantly, because 
the results from one tribe of animals cannot hold quite true for 
another tribe where the organization differs ; but it is so conclusive 
on the significance of the jaws, that I will give, in a translation 
of Professor Rathke’s own words, his remarks on their origin. 
He says, “ That part of the investing mass of the notochord in 
which the basisphenoid is developed in many animals, sends 
out a ‘ray’ or band downwards on each side, which presents 
a remarkable similarity to a rib, not only in its mode of origin, 
but in its original position and form.” These, then, it will be 
seen, are the true epiphyses which correspond to ribs, and, as was 
to be expected, they grow out of the basisphenoid, which was 
the original centrum of the skull; and as the true ribs grow 
down to enclose the posterior part of the digestive organ, so we 
shall see these ribs grow down to embrace its anterior end, and 
become modified into prehensile organs. Professor Rathke goes 
on to say, “ But very early there grows out from near the 
upper end of the ray a long thin process, which passes off at 
an obtuse angle to it and applies itself to the inferior wall of the 
future brain-case.” Thus the ribs, growing down on the diges- 
tive canal, appear to become split, and the upper parts run along 
the top of it and the lower parts run down the sides, thus 
eventually coming to embrace the mouth without bringing it in 
contact with the centrum ; but it ought to be remembered that, 
in the adults of all the animals in which this is observed, union 
has already taken place between the face and the brain-case. 
That the ribs really become split as they apparently do, 
I do not see any reason for believing, and should rather regard 
the upper portions of the forks as connate growths produced by 
causes presently to be considered. The proximal end of the 
cranial representative of a rib ossifies and becomes the quadrate 
bone or incus; an intermediate part becomes the os articulare ; 
while the distal end remains unossified, but developes bones on 
its surface which become the lower jaw. 
