OF THE SKULL IN THE UKODELOUS AMPHIBIA. 
591 
morphological work done by Professor Huxley, have been giving new life to my own 
slow research. 
I venture to offer now a few remarks on the growth and the architecture of the skull 
of vertebrated animals generally. 
Concluding Remarks on the Formation of the SJcull. 
The paired mesoblastic plates, that on each side of the azygous notochord support 
the neural axis, may be considered to be the first foundation of the skeleton. 
Whether the notochord be developed from the underlying hypoblast, or be a median 
tract of mesoblast, does not affect the argument. 
A vertebrated animal may be imagined which should be equally modified, or as little 
modified, at either end. It would be acephalous. 
The least specialization in its sensory nerves, and in the part of the nervous axis from 
which they spring, by which simple sensibility should develop into the power of appre- 
ciating the sapid or odorous qualities of surrounding bodies, or, still further, give to the 
creature some feeling of sound or light, — such an exaltation of the sensory endowment in 
the fore end of the animal would be tantamount to the specialization of the head , as 
distinct from the rest of the body. 
We can conceive of this taking place with but little change in a vermiform animal. 
The endoskeleton may be supposed to remain throughout life as a pair of meso- 
blastic tracts (enclosing the notochord) more solid than the rest of the tissues formed 
from that embryonic layer. The neural axis would be a simple tube, ending in points, 
ampliioxine ; and the hypoblastic tube, or digestive canal, might be as simple as the 
rest of the creature. 
Leaving the actual Amphioxus out of the question, what we see take place in the 
embryos of the Vertebrata generally is a rapid departure from the actual , and a wide 
divergence from any supposed , simplicity of form. 
To get light upon the almost hopeless question of the morphological divergence of 
the head from the trunk we must, in imagination, remove all the factors of cephalic 
specialization, and thus, in idea, reduce the head into the mere fore end of the animal, 
unenlarged, and not transformed, as we actually see it. 
What are those factors X If they can be set in array, and their work and influence 
truly appreciated, we shall be able the better to read the interpretation of this hard 
chapter in morphology. 
The development of nerves of special sense is a correlate of two things first, a large 
regional increase in bulk of the nervous axis ; and, secondly, the budding-out of it on 
each side of special sense-capsules. 
But as the mesoblastic beams on which the nervous axis is laid must grow with the 
growth of that axis, and be modified with its every modification, we have here a great 
factor in this skull-building. 
Moreover, the sense-capsules have to lie in close proximity with their nerve-supply ; 
