S82 ON THE THEORY OF THE VERTEBRATE SKULL 



Vertebrata, outgrowths of the centra unite below to enclose the caudal 

 vessels, and ossify as distinct apophyses. 



If the development of the skull be now compared with that of the 

 spinal column, it is found that (i) the very earliest changes under- 

 gone by the blastoderm in each are almost identical. The primitive 

 groove extends to 'the extremity of the future cranial cavity ; its 

 lateral walls are continuous with the laminae dorsales, and these pass 

 into lamins ventrales, also continuous with those of the spinal region. 

 The lamins dorsales of the head become the cranial walls and enclose 

 the cerebrum — the continuation of the myelon ; the laminae ventrales 

 give rise to the boundaries of the future buccal and pharyngeal 

 cavities. 



2. But at this point the identity of the skull with the spinal 

 column ceases, and the very earliest steps in histological differen- 

 tiation exhibit the fundamental differences between the two. For, 

 in the first place, in no instance save the Amphioxiis, has the 

 notochord as yet been traced through the whole of the floor of the 

 cranial cavity. In no other embryo has it been yet seen to extend 

 beyond the middle vesicle of the cerebrum, or in other words, beyond 

 the level of the rudiment of the infundibulum and pituitary body. 



In the second place, the division into somatomes, in all known 

 vertebrate embryos, stops short at the posterior boundary of the skull, 

 and no trace of such segmentation has yet been observed in the head 

 itself 



3. Apparently as a consequence of these fundamental differences, 

 the further course of the development of the skull is in many respects 

 very different from that of a vertebral column. Chondrification 

 takes place continuously on each side of the notochord, and beyond 

 it, the two trabecule cranii, unlike anything in the spinal column^ 

 extend along the base of the cranium. No distinct cartilaginous 

 centra, and consequently no intercentra, are ever developed. The 

 occipital arch is developed in a manner remotely similar to that in 

 which the neurapophysial processes are formed ; but the walls of 

 the auditory capsules, which lie in front of them, and which give 

 rise to some of the parts, most confidently regarded as neurapophyses. 

 by the advocates of the current vertebral theories of the skull, are 

 utterly unlike neurapophyses in their origin. 



So, if we seek for haemal semi-arcs, we find something very like 

 them, arising from the substance of the basis cranii beneath the 

 auditory cartilage ; but there is none connected with the occipital 

 cartilage, and none with the rudiment of the alisphenoid. The 

 palatopterygoid cartilage might be regarded as the haemal semi-arc 



