438 Royal Society, 



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 

 which, before Von Biir, 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 in- 

 tcrcentra are developed, enclosing the notochord more or less com- 

 pletely. 



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*. 



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 com- 

 position of the ossified cranium, and the vertebrae of an ossified 

 spinal column. But all such inquiries must start with the recogni- 

 tion of the fundamental truths furnished by the study of develop- 

 ment, which, as our knowledge at present stands, appear to me to be 

 summed up in the following propositions : — 



1 . 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. 



2. The basis cranii is never segmented. 



3. The lamina perpend icularis of the ethmoid has the same mor- 

 phological value as the presjjhenoid. 



4. The petrosal has the same morphological value as the mastoid ; 

 if one is not an integral part of the skull, neither is the other. 



5. The nasal bones are not neurapophyses. 



6. The branchial arches have the same morphological value as 

 the hyoid, and the latter as the mandibular arc. 



* 1 feci sure that I met with this phrase somewhere, but I cannot recollect its 

 author. 



