578 ON THE THEORY OF THE VERTEBRATE SKULL 



Thus, I conceive, the study, of the mode in which the skulls of 

 vertebrate animals are developed, demonstrates the great truth which 

 is foreshadowed by a careful and comprehensive examination of the 

 gradations of form which they present in their adult state ; namely 

 that they are all constructed upon one plan ; that they differ, indeed, 

 in the extent to which this plan is modified, but that all these 

 modifications are foreshadowed in the series of conditions through 

 which the skull of any one of the higher Vertebrata passes. 



But if these conclusions be correct, the first problem which I 

 proposed to you, — Are all vertebrate skulls constructed upon a 

 common plan ? — is solved affirmatively. 



We have thus attained to a theory or general expression of the 

 laws of structure of the skull. All vertebrate skulls are originally 

 alike; in all (save AnipJiioxusT) the base of the primitive cranium 

 undergoes the mesocephalic flexure, behind which the notochord 

 terminates, while immediately in front of it, the pituitary body is 

 developed ; in all, the cartilaginous cranium has primarily the same 

 structure, — a basal plate enveloping the end of the notochord and 

 sending forth three processes, of which one is short and median, 

 while the other two, the lateral trabeculae, pass on each side of the 

 space, on which the pituitary body rests, and unite in front of it ; 

 in all, the mandibular arch is primarily attached behind the level of 

 the pituitary space, and the auditory capsules are enveloped by a 

 cartilaginous mass, continuous with the basal plate between them. 

 The amount of further development to which the primary skull may 

 attain varies, and no distinct ossifications at all may take place in it ; 

 but when such ossification does occur, the same bones are developed 

 in similar relations to the primitive cartilaginous skull. But the 

 theory of the skull thus enunciated is not a ' vertebral theory ' ; one 

 may have a perfectly clear notion of the unity of organization of all 

 skulls without thinking of vertebrae. 



So much for the first problem before us. I now proceed to the 

 second question, which was, you will recollect. Given the existence of 

 a common plan of organization of all vertebrate skulls ; is this plan 

 the same as that of a spinal column ? 



To deal properly with this question, we must know what is the 

 plan of organization of a spinal column, and that can be learnt only 

 by a careful study of its development, as well as of its adult modifica- 

 tions. Indeed, the latter are unintelligible without a knowledge of the 

 former. 



It is impossible to form a clear conception of the essential nature 

 of the process of development of a spinal column, or to compare it 



