Likenesses; 07% Philosophical Anatomy 273 



and enclosing the nervous centres along the dorsal region of 

 the frame, then it must be asserted that, in that sense, the 

 skull is in part composed of three bony vertebrae. 



In certain fishes the transition from the spinal column 

 to the skull is so gradual that it is easy to mistake part of 

 that column for part of the skull. Thus, in the sturgeon, 

 the cartilaginous representatives of true vertebrae coalesce 

 into one mass with the cartilaginous skull; and in the 

 Siluroid fish Bagrus the bony vertebrae 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 diflPerences as to development; but, 

 after all, the existence of a remarkable secondary and 

 induced resemblance between these skeletal parts is un- 

 deniable. 



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 

 contrasting with the manifold transverse segmentation of the 

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

 — namely, that in the young eft and the axolotl, before the base 



1 See the paper before referred to, Proceedings of the Zoological Society y 

 1874, p. 196, pi. xxxi., figs. 1 and 2. 



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