ON THE THEORY OF THE VERTEBRATE SKULL 585 



facts, and who view with extreme aversion, any attempt to introduce 

 the phraseology and mode of thought of an obsolete and scholastic 

 realism into biology, will, I think, agree with me, not only in the 

 negative conclusion, that the doctrine of the vertebral composition 

 •of the skull is not proven, but in the positive belief, that the relation 

 'Of the skull to the spinal column is quite different from that of one 

 part of the vertebral column to another. 



The fallacy involved in the vertebral theory of the skull is like that 

 A\-hich, before Von Bar, infested our notions of the relations between 

 fishes and mammals. The mammal was imagined to be a modified 

 :fish, whereas, in truth, fish and mammal start from a common point, 

 -and each follows its own road thence. So I conceive what the facts 

 teach us is this : — the spinal column and the skull start from the 

 same primitive condition — a common central plate with its laminae 

 dorsales and ventrales — whence they immediately begin to diverge. 



The spinal column in all cases becomes segmented into its soma- 

 tomes ; and, in the great majority of cases, distinct centra and 

 intercentra are developed, enclosing the notochord more or less 

 ■completely. 



The cranium never becomes segmented into somatomes ; distinct 

 ■centra and intercentra, like those of the spinal column, are never de- 

 veloped in it. Much of the basis cranii lies beyond the notochord. 



In the process of ossification there is a certain analogy between 

 the spinal column and the cranium, but that analogy becomes weaker 

 and weaker as we proceed towards the anterior end of the skull. 



Thus it may be right to say, that there is a primitive identity of 

 structure between the spinal or vertebral column and the skull ; but 

 it is no more true that the adult skull is a modified vertebral column, 

 than it would be, to affirm that the vertebral column is a modified 

 skull.i 



While firmly entertaining this belief, however, I by no means wish 

 to deny the interest and importance of inquiries into the analogies 

 which obtain between the segments, which enter into the composition 

 -of the ossified cranium, and the vertebra of an ossified spinal column. 

 But all such inquiries must start with the recognition of the funda- 

 mental truths furnished by the study of development, which, as our 

 knowledge at present stands, appear to me to be summed up in the 

 following propositions : — 



I. The notochord of the vertebrate embryo ends in that region of 

 the basis cranii which ultimately lies behind the centre of the basi- 

 .sphenoid bone. 



' I feel sure that I met with this phrase somewhere, but I cannot recollect its author. 



