1864. | Comparative Anatomy and Classification. 541 
lamine, “the dorsal lamine,” grow up, and gradually inclining 
inwards coalesce by their edges i in the middle line. They form the 
foundations of the lateral walls of the skull and spinal eines and 
they assist in enclosing the spaces known as the cranial cavity and 
spinal canal, 
The notochord then becomes surrounded in its whole length by a 
gelatinous investing mass which gives off anteriorly two bands, the 
“trabecula cranii.” These are prolonged forwards, and, according to 
Rathke, embrace the pituitary fossa, and extend as far as the region 
in which tho ethmoid bone is subsequently developed. ‘artilage is 
then formed in this investing mass in by far the greatest majority of 
erania, and this constitutes the cartilaginous base of the skull and the 
bodies of the different spinal vertebre. Mr. Huxley strongly insists 
on the essential difference in the mode in which this chondrification 
of the investing mass takes place in connection with that part of the 
notochord which corresponds to the spinal column, and that which 
lies in the basis cranii. In the former, he states a separate nodule of 
cartilage is developed for each of the bodies of the future vertebrae, 
whilst in the latter a continuous bar of cartilage is formed which 
never exhibits any transverse division or segmentation. And with 
this difference in the mode of chondrification he considers that the 
skull and spine at once begin to diverge from each other in their 
mode of development, each putting on its own special characters, each 
pursuing its own road to its final construction. But we may here 
pause and ask, are our inquiries into the “ history of development” 
so far advanced —established on so sound a basis—as to permit us to 
accept, as unconditionally as Mr. Huxley would wish us to do, the 
primary continuous nature of the cartilaginous bar in the basis cranii 
developed in the investing mass of the notochord? Is its non-seg- 
mented nature to be looked upon as an ultimate fact in development ? 
For our own parts, we doubt much if the subject as yet admits of so 
sweeping a conclusion to be drawn. But if we put altogether on one 
side these doubts which we have just raised, and accept the statement 
asa fact in development, what value are we to attach to it as an 
indication that, at this stage, the skull and spinal column diverge so 
strongly from each other that the one can be no longer regarded as a 
modified form of the other? Is it altogether to outweigh the generally 
admitted fact that the most perfect of all the forms of skull, viz. the 
osseous cranium, exhibits, like the spinal column, in advanced stages 
of its formation, undoubted evidence of segmentation, and in the 
primordial cranium it may be assumed that this segmentation is at 
least potentially indicated? And in the construction and arrange- 
ment of certain, at least, of these segments, there is an approximation 
to the plan of vertebral conformation, more especially in their central 
and neural elements, which at once appeals to the eye of the 
anatomist. 
That the skull in its completely ossified state assumes a very 
definite segmentation is fully admitted by Mr. Huxley, and there runs 
throughout the lectures a very ingenious argument to show that in the 
whole series of osseous crania, from the pike to the man, three origi- 
9 >) 
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