272 LESSONS FEOM NATURE. [CHAP. VIII. 



fish Bagrus the bony vertebra next the head are greatly 

 expanded, and join each other by the same mode of union 

 (by suture) as do true cranial bones ; and this shows how 

 undoubted vertebrae may simulate cranial walls. 



There are, however, various elements which enter into 

 the composition of the brain-case (or skull) which do not 

 enter into that of the spinal-marrow-case (or vertebral 

 column), and there are differences as to development ; but, 

 after all, the existence of a remarkable secondary and induced 

 resemblance between these skeletal parts is undeniable. 



As to development, it has always been affirmed that while 

 the spinal column is essentially, and in almost its earliest 

 stages, a serially segmented structure, the primitive skull 

 presents no serial segmentation. It is indeed true that parts 

 which temporarily or permanently represent in cartilage the 

 bony skull are never serially segmented; and more than 

 this, the cartilaginous precursors of the bones on one side 

 may be completely separated by an interspace of softer 

 substance from their fellows of the opposite side a single 

 fore-and-aft segmentation in the skull thus violently con 

 trasting with the manifold transverse segmentation of the 

 spine. But a most interesting point has lately been noticed* 

 namely, that in the young eft and Axolotl, before the base 

 of the future skull has become cartilaginous, an indication of 

 transverse segmentation is to be traced in the soft tissue of 

 that region a proof of what oversights may be committed by 

 relying too hastily on development as our guide. The con 

 tinuous chondrification of the base of the skull before ob 

 served had led to a denial of all fundamental transverse 

 segmentation of that region by the opponents of the verte 

 bral theory of the skull, while the assertors of that theory 

 regarded such continuity as an induced and adaptive mask 

 ing of a segmentation, visible to the eye of the intellect, 

 though not to that of the sense. The latter view now turns 



* See the Paper before referred to, Proc. Zool. Soc. 1874, p. 196, pi. xxxi. 

 figs. 1 and 2. 



