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 may be 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 vertebre (Poissons Fossiles, t. i. p. 128), M. Agassiz asks, 
“ Ainsi, que seraient dans cette hypothése, le sphénoide principal, les grandes 
ailes du sphénoide, et l’éthmoide, qui forment pourtant le plancher de la 
cavité cérébrale ?— Des apophyses ?—Mais, les apophyses ne protégent les 
centres nerveux que du cdté et d’en haut ?—Des corps des vertébres ?— 
Mais ils se sont formés sans le concours de la corde dorsale ; ils ne peuvent 
done pas étre des corps des vertébres.” (1b. p. 129.) To this it may be 
replied, first that the bodies of the cranial vertebra are not absent; they 
are represented, as above explained, by their cortical portions in the vomer 
(fig. 5, 13), presphenoid (2b. 9) and basisphenoid (2. 5), and by both cortical 
and central portions in the basioccipital (2d. 1): nay, the central part of the 
body of the frontal vertebra is represented in some fishes by the entosphenoid 
(2b. 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 antérieurs’ (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 ‘ éthmoide cranien’ as “un os 
impair, court, de forme presque carré dans lequel sont percés les canaux servant aux nerfs 
_ olfactifs.”—Recherches sur les Poissons Fossiles, t. i. p. 120. 
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