THE THEORY OF THE VERTEBRATE SKULL. 297 



Having concluded this rapid historical sketch of the gradual 

 growth of the true theory of the skull, it may be well if I state, 

 in a brief summary, what I conceive to be the present condition 

 of our knowledge respecting its structure and development : — 



1. All crania result from the modification of the anterior 

 part of that " primitive groove ' of the embryo, the posterior 

 part of which gives rise to the vertebral column ; and, at the 

 very first, there is no discernible difference between that part of 

 the groove which will give rise to the vertebral column, and that 

 from which the skull will be produced. 



2. The first changes which take place, in both the cranial 

 and the spinal regions of the primitive groove, are also pre- 

 cisely similar, the dorsal laminae growing up and uniting 

 together in the middle line, so as to enclose a cavity which is, 

 on the one hand, the primordial brain-case, and, on the other, 

 the primordial spinal canal. So far, a unity of organization may 

 be predicated of both brain-case and spinal canal ; but the brain- 

 case is not yet a skull, nor the spinal canal a vertebral column. 



3. Beyond this point, the course of development of the 

 cranial region differs absolutely from that of the spinal region. 

 In the latter, that histological differentiation takes place which 

 results in the formation of the proto-vertebrae, while in the 

 skull no such process occurs. Again, the notochord extends 

 throughout the whole length of the spinal column ; while, as 

 soon as the skull is distinguishable, as such, the notochord 

 ceases to extend beyond the middle of its floor, stopping im- 

 mediately behind that part which lodges the pituitary fossa.* 



4. Furthermore, when chondrification takes place in the 

 spinal column, separate masses of cartilage are developed in 

 each proto-vertebra ; but, when chondrification commences 

 in the base of the skull, it gives rise to a continuous body of 

 cartilage, which never exhibits any trace of transverse division, 

 or segmentation ; but is always divided under the pituitary body 

 into two longitudinally-arranged crura, the " trabecular cranii" 



5. Hence it follows that, though the primordial brain-case 

 and the primordial spinal canal are identical in general j^lan of 

 construction, the two begin to diverge as soon as the one puts 



* Amphioxus forms an exception, probably only apparent, to this generalization. 



