r.ai 



centres, I have termed * autogenous elements.' " The remaining 

 elements, which he classes as " exogenous," because they " shoot 

 out as continuations from some of the preceding elements," are the 

 diapophyses diverging from the upper part of the centrum as the 

 parapophyses do below, and the zygapopliyses which grow out of '.he 

 distal ends of the neurapophyses and hsemapophyses. 



If, now, these are the constituents of the vertebrate segment is 

 its typical completeness ;" and if the vertebrate skeleton consists ol 

 a succession of such segments ; we ought to have in these con- 

 stituents, representatives of all the elements of the vertebrate 

 skeleton — at any rate, all its essential elements. Are we then to 

 conclude that the " diverging appendages," which Professor Owen 

 regards as rudimental limbs, and from certain of vrhich he considers 

 actual limbs to be developed, ai-e typically less important than some 

 of the above-specified exogenous parts — say the zygapophyses ? 



That the meaning of this question may be understood, it will be 

 needful briefly to state Professor Owen's theory of The Nature oj 

 JAmhs ; and such criticisms as we have to make on it must be in- 

 cluded in the parenthesis. In the first place, he aims to show that 

 the scapular and pelvic arches, giving insertion to the fore and hind 

 liobs respectively, are displaced and modified haemal arches, 

 originally belonging in the one case to the occipital vertebra, and in 

 the other case to some trunk-vertebra not specified. In support of 

 this assumption of displacement, carried in some cases to the extent 

 of twenty-seven vertebrse. Professor Owen cites certain acknow- 

 ledged displacements which occur in the human skeleton to the ex- 

 tent of half a vertebra — a somewhat slender justification. But for 

 proof that such a displacement has taken place in the scapular arch, 

 he chiefly relies on the fact that in fishes, the pectoral fins, which 

 are the homologues of the fore-limbs, are directly articulated to 

 certain bones at the back of the head, which he alleges are parts 

 of the occipital vertebra. This appeal to the class of fishes is 

 avowedly made on the principle that these lowest of the Vertebrata 

 approach closest to archetypal regularity, and may therefore be 

 expected to show the original relations of the bones more nearly, 

 Simply noting the facts that Professor Owen does not give us any 

 Iransitional forms between the alleged normal position of the 

 scapular arch in fishes, and its extraordinary displacement in the 

 higher Vertebrata ; and that he makes no reference to the embryonic 

 phases of the higher Vertebrata, which might be expected to ex- 

 hibit the progressive displacement ; we go on to remark that, in 

 the case of the pelvic arch, he abandons his principle of appealing 

 to the lowest vertebrate forms for proof of the typical structure. 

 In fishes, the rudimentary pelvis, widely removed from the spinal 

 column, shows no signs of having belonged to any vertebra; and 

 iiorc Professor Owen instances the perennibranchiate Batracliia aa 



