144 Zoological Society. 



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 Metamorphose des 

 Batraciens, Ann. des Sci. Nat. No. xviii. p. 401) ; and also the 

 invariableness of the number of the vertebrae in the Mammalian's 

 neck, resulting from the presence of the same number of nerves, and 

 irrespective of the length of the vertebrae. 



Section V. 



A segment is the representative in the Articulate of a vertebra in 

 the Vertebrata. 



This view has been advocated by Geoffroy St. Hilaire, both in his 

 " Memoire sur la Vertebre," in the ninth volume of the ' Memoires du 

 Museum 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 ut antea, 256). Thus if we suppose a vertebra stopped in 

 the first stage, and forming the external instead of the mternal sup- 

 port of the body, we have a segment of an articulate creature, with 

 only an histiological difference, which must by no means be allowed 

 to conceal from us the true nature of a part (Geoffroy St. Hilaire, 

 Sur la Vertebre, ut antea, p. 92). 



3. If to this view it should be objected, that the including in the 

 one case what is excluded in the other dispels all semblance of homo- 

 logy, it must be answered — 



«. That notwithstanding this difficulty, the general homology of 

 the vertebrate and articulate skeletons as wholes has long been ad- 

 mitted, though this more particular one of their parts has not been. 



/3. That the haemal arch of the Vertebrata, whose normal office it 

 is to enclose the main blood-vessels of the body, and which office it 



I 



