Page:Proceedings of the Royal Society of London Vol 69.djvu/18

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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.

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.