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vertebra and a certain three vertebrse with the features of a certain 

 two in the normal spine is more to the point. If we knew what the 

 precise number of original human vertebrae is, and if they all came 

 to maturity, a departure from that number could be ascribed to irre- 

 gular segmentation. But these conditions do not exist. Normally there 

 are respectively seven, twelve and five vertebrae in each of the prsesacral 

 regions, and this whole prsesacral portion constitutes a physiological 

 unit as above stated. It is possible to say that in some cases thirteen 

 vertebrae from the 8th to the 20th inclusive assume the peculiarities 

 of thoracic vertebrae and that the regions before and behind them 

 develop very nearly as usual, with their normal number of segments 

 respectively. Of course what is said of thirteen thoracic vertebrae may 

 be said, nautandis mutatis, of eleven. Irregular segmentation may have 

 occurred. It may be said on the other hand that the ilium in its 

 migration tailwards did not join the 25th but passed beyond it, or 

 stopped short of it, adaptation occurring subsequently. Either theory 

 is supposable; neither excludes the other; both may be true. What 

 actually occurs must be settled, not by argument, but by observation. 

 Now Bardeen states distinctly that "the thoracic vertebrae are diife- 

 rentiated from the others at this early period", that is at the time 

 the ilium joins the spine at about the end of the fifth week before 

 chondrofication has begun. He has seen eleven thoracic and six lumbar 

 vertebrae in embryo XVII, the probable age of which is six weeks. It 

 is highly probable that further observations will show actual increase 

 or diminution at this age without compensation. Bardeen has found 

 that during the blastemal stage the costal elements of the thoracic 

 vertebraae develop much more freely than those of other regions. This, 

 therefore, has occurred before the point at which the ilium will stop 

 in its migration has become apparent. Consequently an increase or 

 a decrease in the number of thoracic vertebrae is not necessarily 

 determined by that fact. Under these circumstances it does not seem 

 absurd to call such a variation the effect of an error in segmentation, 

 meaning thereby that a larger (or smaller) number of vertebrae develop 

 ribs independently of the position of the ilium. This structure may 

 move tailwards along precisely the same number of vertebrae as usual ; 

 only there is an unusual number before it. But one must agree with 

 Bardeen that "differentiation in the post-thoracic region depends ap- 

 parently in the main upon the position of the posterior limb". His con- 

 clusion that "it seems fair to assume that the primitive vertebrae become 

 differentiated according to the demands of their environment" may be 

 accepted without denying other causes. 



