of the Pelvic Ple,vm in Acanthias vulgaris. 25 



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. 



BIBLIOGRAPHY. 



(1.) T. Davidoff, M., ' Beitrage zur vergleichenden Anatomic der hiuteren 



Gliedmasse der Fische. Morphol. Jahrb.,' vol. 5, 1879. 

 (2.) Day, F., 'The Fishes of Great Britain and Ireland,' 2 TO!S., London, 



188084. 



(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- 



pterygium," 'Anat. Heft e,' vol. 1. 1894. 

 (6.) Gadow, H., and Abbott, E. C., " On the Evolution of the Vertebral Column 



of Fishes," 'Phil. Trans.,' B, vol. 186, 1895. 

 (7.) Bumpus, H. C., "A Contribution to the Study of Variation," ' Joiu-n. 



Morph.,' vol. 12, 1897. 

 (8.) Fuerbringer, M., ' Uber die spino-occipitalon Nerven der Selachier, u.s.w.,' 



Leipzig, 1897. 

 (9.) Waite, F. C., "Variations in the Bracliial and Lumbo-sacral Plexi of 



Nectums maculosus." ' Bull. Mus. Comp. Zool., Harvard, Cambridge, 



Mass.,' vol. 31, 1897. 

 (10.) Braus, H., "Uber die Innervation der paarigen Extreinitaten bei.Selachiern, 



u.s.w.," 'Jena. Zeit.,' 1897. 



