526 



the human foetal skeleton, and the separate bones of the adnlt skeletons of 

 inferior animals, are pregnant with interest, and rank among the most striking 

 illustrations of unity of plan in the vertebrate organization." 



It is true that on the following page he seeks to explain this 

 seeming contradiction by distinguishing 



"between those centres of ossification that have homological relations, and 

 those that have teleological ones i.e., between the separate points of ossiiica- 

 tton of a human bone which typify vertebral elements, often permanently dis- 

 tinct bones in the lower animals ; and the separate points which, without such 

 signification, facilitate the progress of osteogeny, and have for their obvioua 

 final cause the well-being of the growing animal." 



But if there are thus centres of ossification which have homo- 

 logical meanings, and others which have not, there arises the ques- 

 tion How are they always to be distinguished ? Evidently in- 

 dependent ossification ceases to be a homological test, if there are 

 independent ossifications that have nothing to do with the homo- 

 logies. And this becomes the more evident when we learn that 

 there are cases where neither a homological nor a teleological 

 meaning can be given. Among various modes of ossification of the 

 centrum, Professor Owen points out that " the body of the human 

 atlas is sometimes ossified from two, rarely from three, distinct 

 centres placed side by side " (p. 89) ; while at p. 87 he says : " In 

 osseous fishes I find that the centrum is usually ossified from sis 

 points." It is clear that this mode of ossification has here no home- 

 logical signification ; and it would be difficult to give any teleo- 

 logical reason why the small centrum of a fish should have more 

 centres of ossification than the large centrum of a mammal. The 

 truth is, that as a criterion of the identity or individuality of a bone, 

 mode of ossification is quite untrustworthy. Though, in his " ideal 

 typical vertebra," Professor Owen delineates and classifies as sepa- 

 rate " autogenous " elements, those parts which are " usually 

 developed from distinct and independent centres ;" and though by 

 doing so he erects this characteristic into some sort of criterion ; 

 yet his own facts show it to be no criterion. The parapophyses 

 are classed among the autogenous elements ; yet they are auto- 

 genous in fishes alone, and in these only in the trunk vertebras, 

 while in all air-breathing vertebrates they are, when present at all, 

 exogenous. The neurapophyses, again, " lose their primitive in- 

 dividuality by various kinds and degrees of confluence:" in the 

 tails of the higher Vertebrata they, in common with the neural 

 spine, become exogenous. Nay, even the centrum may lose its 

 autogenous character. Describing how, in some batrachians, 

 " the ossification of the centrum is completed by an extension of 

 bone from the bases of the neurapophyses, which effects also the 

 coalescence of these with the centrum," Professor Owen adds : 

 " In Pelobates fuscus and Pelobates cultripes, Miiller found the en- 



