10 Mr. E. C. Punnett. On the Composition and Variations 



IV. Asymmetrical Variations. 



In five out of the hundred cases examined the pelvic girdle was 

 found to be asymmetrically placed (vide Table I, Nos. 10, 64, 95, 99, 

 100), that is to say, the serial number of the first girdle-piercing nerve 

 differed on the two sides. I have already shown that the same 

 phenomenon occurs in Mustehis lewis ( (12), pp. 335 and 343), and for 

 other groups it has been noticed more especially by Paterson ( (3), 

 p. 522) in the case of man, and by Bumpus ((7), p. 465), and "Waite 

 ( (9), p, 79) in the case of Ncdurus. The last two writers suggest 

 that in Xe'lurus such asymmetry may be due to the fact that the 

 curvature of the developing Amphibian embryo is lateral, seeking to 

 account in this way for the large number of asymmetrical variations 

 in the group, which amounts to 8 per cent. Much stress, however, 

 cannot be laid on this, as Paterson has shown that asymmetrical 

 variations reach as much as 8 3 per cent in man, whilst among the 

 Elasmobranch the percentage reaches 5 per cent, both in Mustelus and 

 Acamthias. Both these last groups differ from Necturus in that the 

 embryonic curvature is dorso-ventral instead of lateral. 



V. Vitiations in the Number and Position of the Nerve Canal*. 



It has already been stated that there may be either, one or else two 

 nerve canals in the pelvic girdle. These variations may be arranged 

 in three classes fig. 3, A, B, and C. 



FIG. 3. 



C. 



