OCT. 1895. VERTEBRAL COLUMN OF AMIA HAY 49 



ganze kiinftige Gestaltung des Wirbelkorpers maasgebende Einrich- 

 tung zu Stande gekommen." It is not impossible, however, that we 

 have here, after all, no new structures, but, as in so many other 

 cases, only new adaptations and new combinations of structures 

 already well known in lower forms, 



To what extent the bases of the lower arches, the hypocentra, 

 may be represented in the trunk region of the Urodeles is questiona- 

 ble. Possibly with the disappearance of the lower series of ribs the 

 bases of the arches also have totally disappeared. This seems to be 

 the condition of the Anura, certainly in the case of those with epi- 

 chordal vertebrae. But evidences of hypocentral ossifications should 

 be looked for in the Urodeles in the trunk, and such ossifications are 

 certainly present in the tail. 



The opinion of Prof. Cope that the vertebral centra of the Am- 

 phibians are represented by only the hypocentra was doubtless the 

 outcome of the idea that only pleurocentra and hypocentra enter into 

 the composition of centra. That the bases of the upper arches and 

 haemacentral bones may also participate in the composition, there can 

 now be no doubt. 



In the " Biologisches Centrallblatt," Band vi., 1888, Dr. G. Baur 

 has discussed the morphogeny of the vertebral column and given the 

 history of the question and the literature bearing on the subject. The 

 view proposed first by Cope, and accepted by Albrecht, Dollo and 

 Baur, that the vertebral centrum of Amniota has been derived from 

 the pleurocentrum is, I believe, correct. The evidences in favor of 

 this hypothesis have been presented by Baur in the paper just cited. 

 A consideration of the situations in which the various elements con- 

 nected with and forming the vertebra confirms the hypothesis. 



The arches, upper and lower, of the Amniota are unquestion- 

 ably homologous with those of the lower vertebrates, and, as in the 

 latter, are developed in the intersection of two sets of membranous 

 septa, viz. : those between the myomeres and those between the two 

 sides of the body, including in the latter the subperitoneal membrane. 

 The hypocentra, therefore, are cut by the transverse septa. The 

 intercalated cartilages, or pleurocentra, on the other hand, fall be- 

 tween the septa and in the myorneres. When the hypocentrum enters 

 into the composition of the vertebral centrum, the lower arches will 

 naturally be found intimately connected with the centrum. And such 

 is its position in all the living tailed amphibians (Stannius, Am- 

 phibia, page n) being attached to the middle or hinder half of the 

 centrum. In the fossil Hylonomus fritschi, Credner, however, the 

 lower arches are represented as being attached between two contig. 



