ON THE VERTEBRATE SKELETON. 323 



well as every character determining that of the surrounding vertebral elements, 

 are to be nullified and set aside ! M. Agassiz, moreover, seems not to have 

 suspected that the notochord may have other and more immediate and import- 

 ant functions than even those relating to the vertebral column. The peculiar 

 elective attraction of its component cells for the gelatinous principle maybe es- 

 sential to the due operation of those neighbouring cells which form the basis of 

 the neural axis, and which as exclusively assimilate the albuminous principle : 

 and this reciprocal antagonism in the selection of particular proximate prin- 

 ciples from the common primitive blastema may explain the contemporaneous 

 origin of notochord and myelon in the embryonic trace, when all development 

 is as yet the work of cell-assimilation and metamorphosis, without any supply 

 from a vascular system, this being a later formation in the building up of the 

 organic machinery. By confining, however, his views of the notochord to one 

 of its functions in relation to a single vertebral element, and by extending his 

 conclusions from this to the entire vertebra, M. Agassiz, though recognising 

 more absolutely than Cuvier, the vertebral character of the neural arch of 

 the occipital segment, concludes that Nature discards that type in the con- 

 formation of the cinctures that precede it and which successively girt the 

 mesencephalon, prosencephalon and rhinencephalon. 



Assuming a gratuitous explanation of the hypothetical absence of the bodies 

 of the cranial vertebrae (Poissons Fossiles, t. i. p. 128), M. Agassiz asks, 

 " Ainsi, que seraient dans cette hypothese, le sphenoide principal, les grandes 

 ailes du sphenoide, et l'ethmoide, qui forment pourtant le plancher de la 

 cavite cerebrale ? — Des apophyses ? — Mais, les apophyses ne protegent les 

 centres nerveux que du cote et d'en haut ? — Des corps des vertebres ? — 

 Mais ils se sont formes sans le concours de la corde dorsale ; ils ne peuvent 

 done pas etre des corps des vertebres." (lb. p. 129.) To this it may be 

 replied, first that the bodies of the cranial vertebrae are not absent ; they 

 are represented, as above explained, by their cortical portions in the vomer 

 (fig. 5, 13), presphenoid (ib. 9) and basisphenoid (ib. 5), and by both cortical 

 and central portions in the basioccipital (ib. 1) : nay, the central part of the 

 body of the frontal vertebra is represented in some fishes by the entosphenoid 

 (ib. 9'), which remains distinct from the cortical part below, as does the central 

 part of the body of the atlas in the siluroid fish. If it were true, indeed, 

 that the entosphenoid was pierced by the canals transmitting the olfac- 

 tory nerves*, Bojanus' idea of its general homology as the centrum of the 

 ' vertebra optica ' must be abandoned. But the parts called ' olfactory 

 nerves' by M. Agassiz, pass from the prosencephalic to the rhinencephalic 

 compartments of the cranium not merely above the bone called ' cranial 

 ethmoid ' by the same author, but, also, through the upper part of the inter- 

 space between the bones (orbitosphenoids) which the entosphenoid (9') 

 sustains: and the true olfactory nerves perforate the neurapophyses (14) 

 which Bojanus called ' ethmoid ' and which Cuvier and M. Agassiz have 

 termed 'frontaux anterieurs' (see ante, pp. 214-226). The alisphenoids, being 

 notched or perforated by their proper intervertebral nerves, are ' apophyses' 

 (neurapophyses), and accordingly do protect the sides of their proper nervous 

 centre, the mesencephalon. The central jelly-cells of the notochord appear to 

 be withdrawn into the occipital region before ossification of the basisphenoid 

 commences, and that modified vertebral body is therefore developed at the 

 expense of the fibrous sheath of the notochord, and is represented by its 

 ' cortical ' part only. But its general homology is determined by its con- 



* M. Agassiz has described this bone under the name of ' ethmoide cranien ' as " un os 

 impair, court, de forme presque carre dans lequel sont perces les canaux servant aux nerfs 

 olfactifs." — Recherches surles Poissons Fossiles, t. i. p. 120. 



