425 



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 '- 1 -"' 

 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 modifi- 

 cations. 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*. 



1 . The primary changes of form exhibited by the blastoderm in 

 the region of the spinal column, are, in all the Vertebrata whose 

 development has yet been studied, precisely the same. Two ridges, 

 the " laminae dorsales," bounding a narrow elongated groove, rise 

 up and eventually unite with one another so as to enclose a cavity 

 the neural canal. External to the junction of the laminae dorsales 



* See Note VI. for the details of the development of the spinal column in 

 Vertebrata generally. 



