DEVELOPMENT OP THE SKULL IN STURGEONS. 
177 
addition, itself, to the neural axis, then we have to reconsider the meaning of any 
and every part of the cranial skeleton which may be formed in front of the notochord 
and of the overlying and overfolded mid-brain. 
Thus all the jprochorclal cranial, and all the pre-maoidibida.r facial growths of the 
skeletal cartilage have to be subjected, again, to the severest morphological inquisition 
under that light—the light of Embryology—which alone can make manifest their 
true meaning. 
If the notochord be the true organic axis of the skeleton of the animal, and if the 
visceral (or branchial) arches were only developed in relation to the hypoblastic bran¬ 
chial outgrowths of the dilated respiratory pharynx, then it seems to be necessary that 
we should consider all the skeletal parts in front of those structures as superadded 
specializations of, or “ outgrowths ” from, the front end of the proper Vertebrate form. * 
In certain types, namely, the Lamprey, the Selachians, the Urodela, and the Anura, 
the trabeculae are chondrified before the hinder or parachordal tracts. Nevertheless, 
the posterior part of the trabeculae in them is parachordal; the rest, up to the “ atlantal 
region, may afterwards chondrify separately, as in the Urodela, or continuously, as in 
the others. 
In Acipenser, Lepidosteus, and Salmo I find no difference of time in this matter, and 
should therefore be inclined to look upon the earlier growth of the trabeculae as due to 
the special weight and pressure of the fore-brain in those cases, and as a non-essential 
modification, just as I consider the later segmentation of the trabeculae from the 
investing mass in Scdmo and Chelone , and to a less extent in Crocodiles and Birds, as 
a non-essential specialization. 
Yet the trabeculae are as truly part of the proper mesoblastic axial skeleton as the 
fore-brain is part of the proper epiblastic neural axis. 
The whole of the pre-cerebral tracts of the skull, namely, the cornua trabeculae and 
the greater part of the intertrabecula—all of it that lies in front of the exit of the 
olfactory nerves—I should consider to be mere outgrowths or “ apophyses ” of the 
cranial skeleton. 
The axial skeleton is more aborted in front than the axial nervous system, even if 
the fore-brain is reckoned as an additional part, for the mid-brain bends completely 
over upon itself, and the notochord only partially. Yet, as I have shown in Chelone 
viridis, its cartilaginous mesoblastic sheath is continued downwards as solid cartilage 
below and in front of the end of the notochord. I am inclined to think that the 
intertrabecula is a breaking out again of that mesoblastic tract (a sort of cranial 
“ spadix ”) but with its continuity, for a short space, interrupted. 
The ventral part of the primordial skull presents as many difficulties as the dorsal; 
the post-oral visceral (or branchial) arches, which may be both superficial and deep, 
* Dor my own part, I am quite content that this should be so, albeit my own descriptive language will 
have to undergo a considerable amount of evolutional modification, and many things that seemed to me, 
once, to be clear and certain, made dark and uncertain. 
2 A 
MDCCCLXXXIL 
