WAITE: PLEXI OF NECTURUS. 87 
cephalad. Such general displacement caudad 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 unsymmetrical relations of the appendages are 
more frequently recorded in Amphibia than in Reptilia, 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 sd 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 unsymmetrical 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. 
Von 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 unsymmetrical, it 
involves loss or addition of a spinal nerve on one side only. 
Such a scheme, by reason of its apparent artificiality, fails to appeal 
to me strongly, and no conditions found in Necturus 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- 
