20 
the number of pairs of nerves given off. When more than one pair 
perforate one piece of bone, it results from an anchylosis of several 
vertebrae, as in the sacrum; and the coccygeal vertebree, which ap- 
pear to be an exception to the definition, are not so in reality, the 
spinal cord passing into them in the fcetal condition, and being gra- 
dually withdrawn just in the same manner as is the case in some of 
the Coleoptera. As is clearly seen in them, too, the cauda equina 
represents the nerves of the vertebree from which the cord has been 
withdrawn. Some Vertebrata, as e. g. the Python, retain the original 
relation of the vertebree and centres throughout the whole of the 
spinal cord (Owen, Report ut antea, 221). 
2. The same dependence of the vertebrae on the nervous centres 
is shown by the fact, that the tail which is reproduced by Lizards, 
in the case of the loss of that member, is a single bone, because 
although bone may be reproduced, the spinal cord cannot be (Owen 
ut antea, 254). 
3. In accordance with this definition may also be cited the very 
long vertebra which is formed on that part of the spinal cord of the 
Anourous Batrachians which does not give off nerves, and which is 
not the result of anchylosis of several elements, but arises from one 
point of ossification (Martin St. Ange, Recherches anatomiques et 
physiologiques sur les Organes transitoires et la Métamorphose des 
Batraciens, Ann. des Sci. Nat. No. xviii. p. 401); and also the 
invariableness of the number of the vertebree in the Mammalian’s 
neck, resulting from the presence of the same number of nerves, and 
irrespective of the length of the vertebre. 
Secrion V. 
A segment is the representative in the Articulata of a vertebra in 
the Vertebrata. 
This view has been advocated by Geoffroy St. Hilaire, both in his 
“‘ Mémoire sur la Vertébre,’’ in the ninth volume of the ‘ Mémoires du 
Muséum d’ Histoire Naturelle,’ and previously in a memoir read by 
him before the Academy in 1820. Nevertheless, the argument on 
which I would mainly rest it, is not yet universally admitted, for we 
find M. Emile Blanchard very recently asserting that nothing really 
indicates the analogy between the spinal cord of the Vertebrata and 
the ganglia of the Articulata. 
1. We have seen what a close relation of correspondence exists in 
the Articulata between the segments and the ganglionic nervous 
centres ; and we have endeavoured to prove that in the Vertebrata a. 
vertebra is the correlative of one of the spinal nervous centres ; and 
also that the spinal cord of the one class is the representative of the 
ganglionic cord of the other; whence it appears, that a segment of 
the Articulata and a vertebra of the Vertebrata must be homologous. 
2. The ossification of the centrum of a true vertebra is first peri- 
pheral, and subsequently fills up the interior with osseous matter 
(Owen wt antea, 256). Thus if we suppose a vertebra stopped in 
the first stage, and forming the external instead of the internal sup- 
