432 lluynl Sociely .— 



But if propositions of this generality can be enunciated with regard 

 to all bony vertebrate skulls, it is needless to seek for further evidence 

 of their unity of plan. These propositions are the expression of that 

 plan, and might, if one so pleased, be thrown into a diagrammatic 

 form. There is no harm in calling such a convenient diagram the 

 ' Archetype ' of the skull, but I prefer to avoid a word whose con- 

 notation is so fundamentally opposed to the spirit of modern science. 



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 cor^prehensive 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 Vertehrata passes. 



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

 posed — 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 Amphioxus ?) 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 trabeculse, 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 tliern. 

 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. 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 colunm ; and that can be learnt only by a 

 careful study of its development, as well as of its adult modifications. 

 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 with that of the skull, unless we analyse very carefully, and di- 

 stinguish from one another, the successive steps of that process. 



