The facts recorded have been used as criteria between the two rival
theories of limb origin with the following results :
(1.) To explain the variations on the side-fold excalation theory, it must be assumed that excalation of segments is going on in the collector and pre-collector areas, whilst, at the same time, intercalation is taking place in the post-girdle area ; or, in other words, that the portion of the vertebral column in front of the girdle is tending to split up into fewer segments, whilst simultaneously that portion behind the girdle is tending to become divided into more segments. Leaving on one side the improbability of two contiguous portions of the vertebral column undergoing at the same time two opposite processes, an ex- amination of the number of whole vertebrae associated with different positions of the girdle lends practically no support to the view that intercalation is going on in this area.
(2.) On the side-fold excalation theory an explanation of the varia- tions in the position and number of the nerve canals of the girdle and of the occasional instances of asymmetry, necessitate the assumption that the pelvic girdle in different specimens is not homologous an assumption which at present seems unjustifiable.
(3.) The different variations observed are not discordant with the view that the limb is capable of migrating along the body, on which view it must be supposed that a secondary rostral migration has followed a primary caudal one. Moreover, such a view receives con- firmation from the existence of a posterior collector, and of ;t more extensive anterior collector in certain embryonic stages.
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(1.) T. Davidoff, M., ' Beitrage zur vergleichenden Anatomic der hiuteren Gliedmasse der Fische. Morphol. Jahrb.,' vol. 5, 1879.
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(3.) Paterson, A. M., " The Human Sacrum," 'Proc. Roy. Soc.,' vol. 51, 1892. (4.) Bateson, W., ' Materials for the Study of Variation,' London, 1894. (5.) Mollier, S., " Die Paarigen Extremitaten der Wirbelthiere, I. Das Ic-hthyo-
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