waite; plexi of necturus. 87 



cephalad. Such general displacement cai;dad is capable of interpreta- 

 tion either as atavistic, or as indicating some force at work tending to 

 lengthen the vertebral series. 



It is noticeable that uusymmetrical relations of the appendages are 

 more frequently recorded in Amphibia than in Keptilia, Aves, or Mam- 

 malia. This may possibly be due simply to the probably greater num- 

 ber of Amphibia examined. But assuming that this is not the case, it 

 is not clear why such variation should exceed in this class. I offer, 

 however, the suggestion that it may be due to the position of the em- 

 bryo during development. In Amphibia the position is such that the 

 embryo is curled within the egg membrane laterally, which would tend 

 to shorten the concave side, and so might induce a displacement of the 

 appendage of that side cephalad or caudad. In the other groups men- 

 tioned, the curling of the embryo is dorso-ventrally, and would give less 

 tendency to uusymmetrical relations. 



As to the second question involved, — Is there any correlation be- 

 tween the variations of the plexus and those of the girdle ? — there is a 

 wide difference of opinion, ranging from very close correlation, as main- 

 tained by Ruge ('93, p. 393), to no correlation, as believed by von Ihering 

 ('78), who regards nerve and skeleton as independent in variation. 

 Yon Ihering holds that the plexus migrates as a whole independently of 

 the girdle ; if cephalad, then a presacral pair of spinal nerves drops out, 

 if caudad, a pair is inserted, and if the migration be uusymmetrical, it 

 involves loss or addition of a spinal nerve on one side onh^. 



Such a scheme, by reason of its apparent artificiality, fails to appeal 

 to me strongly, and no conditions found in Xecturus warrant adoption 

 of such a hypothesis. 



The plexus is a combination of spinal nerves, the nature of whose 

 topography is secondary. The point which needs emphasis in this con- 

 nection is that the variation of the plexus is not a variation as a whole, 

 but the summation of variations of its elements (individual spinal nerves). 

 The variation in strength of these elements is probably a response to the 

 influence from their end organs, — the muscles, — for there appears to 

 be direct correlation between the size and activity of a muscle, and the 

 strength of the nerve or nerves supplying it. "Whether the muscles of 

 a given metamere shall develop as limb muscles, i. e. muscles of increased 

 size and activity, or into less extensive trunk musculature, depends upon 

 whether or not in that region an appendage — the primary determinant 

 of both plexus and girdle — is to arise. If appendicular musculature 

 develop, the increased activity presumably induces stronger develop- 



