322 report— 1846. 



too successfully in regard to the rising generation of anatomists, to be 

 obscured. Ideas and statements are misquoted, unintentionally, doubtless, 

 and through neglect of reference to the original work (as in the citation of 

 the bones representing the bodies of the cranial vertebrae in the Okenian 

 theory): or they are misunderstood (as where the arches, neurapophyses or 

 ' bogentheile,' composed as Oken truly said by the alisphenoids and orbito- 

 sphenoids are held to be synonymous with the ' plaques protectrices ' of M. 

 Vogt) : the most extreme and least defensible views are selected out of each 

 tentative step in the inquiry, and are clubbed together to represent the 

 general result, which is of course dismissed with as sweeping a condemnation. 

 The specific objections raised by Cuvier are deemed well-founded and un- 

 assailable ; and to these M. Agassiz adds the following. Premising that, 

 " the formation of vertebrae presupposes as a first condition the existence 

 of a notochord*;" and, arguing upon this basis, and on the assumption 

 that the cephalic extension of the ' chorda dorsalis' as it is permanently 

 manifested in the Branchiostoma is not so great in the embryos of other and 

 higher fishes, but is arrested at the region of the alisphenoid from the com- 

 mencement of its development, M. Agassiz concludes: — "Now, the application 

 of this principle to the composition of the skull demonstrates at once that there 

 exists but one cranial vertebra, the occipital vertebra, and that the rest of 

 the skull is foreign to the vertebral system-]-." 



At the period of development described and figured by M. Vogt in the em- 

 bryo of the Coregonus, which period M. Agassiz conceives to represent theveiy 

 earliest condition of the anterior extremity of the notochord, the pointed ex- 

 tremity of the gelatinous central cells of this part terminates at the posterior 

 boundary of the hypophysial space: but the peripheral capsule of the notochord 

 extends over that space and forwards to the obtuse anterior extremity of the 

 embryonal ' basis cranii ' : and it is in the expanded aponeurosis, directly con- 

 tinued from the chorda along the basis cranii, that the thin stratum of carti- 

 lage cells are developed, arching along the sides of the hypophysial space, 

 from which the ossification of the basisphenoid, presphenoid and vomer 

 proceeds %. 



The superaddition or the later continuation of the cylindrical gelatinous 

 'chorda' in the aponeurotic basis of the cartilaginous and osseous growths of 

 the vertebral centres in the trunk, seems to relate chiefly to their more or 

 less cylindrical form in that region : the notochord regulates, as a mould, the 

 course of ossification, disappearing by absorption as the bony lamellae of the 

 vertebral bodies encroach upon it in their centripetal progress: the notochord 

 plays an important part also in the establishment of the elastic jelly-filled 

 capsular joints in the back-bone of fishes ; and therefore it might well be 

 dispensed with, or be early and rapidly removed, in the development of the 

 flattened, expanded and anchylosed or immoveably articulated bodies of the 

 cranial vertebrae. And, besides, the notochord is immediately concerned in 

 the development of only one of the elements of the typical segment of the 

 endoskeleton. It is obviously, therefore, an unwarrantable and erroneous 

 application of a developmental character, to conclude, from a modifica- 

 tion of this one character in respect of a single element, the 'centrum,' that 

 every other character establishing the general homology of such element, as 



* " La formation des vertebres suppose, comme premiere condition, l'existence d'une 

 ' corde dorsale.'" — Op. cit. torn. i. p. 127, livr. xviii. (1843.) 



•f- " Or, l'application de ce principe a la composition de la tete nous moutre d'entree qu'il 

 n'existe gu'une seute vertebre crdnienne, la vertebre occipitale, et que le reste de la tete est 

 etranger au systeme vertebrale." — lb. p. 127. 



J Hunterian Lectures on Vertebrata, 1846, p. 71. 



