Variations of the Pelvic Plexus in Acanthias vulgaris. 141 



(c) The number of nerves forming the collector ; 



(d) The number and position of the nerve canals ; 



(e) The number of the fin rays ; 



(f) The number of the whole vertebrae. 



(2) Asymmetry occurred in an appreciable number of cases. 



(3) Differences occurred in the two sexes on the following points : 



The position of the girdle is more rostral in the male than in 

 the female. The post-girdle fin innervation area is greater in 

 the male than in the female, owing to the development of the 

 mixipterygium. 



(4) The female is, on the whole, more variable than the male. 



(5) A well-marked correlation exists between — 



(a) The position of the girdle and the number of collector 

 nerves ; 



(b) The position of the girdle and the number of post-girdle 

 nerves ; 



(c) The position of the girdle and the number of whole 

 vertebrae. 



(6) No correlation was found between the number of the fin rays and 



the number of fin nerves. 



(7) At certain stages in ontogeny the number of collector nerves is 



greater than in the adult. 



(8) At certain stages in ontogeny the number of post-girdle nerves 



is greater than in the adult. The most caudal two or three of 

 these form a posterior collector — a structure which is never 

 found in the adult. 



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 examination 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, necessitates the assumption 



M 2 



