78 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
sacrum, to nerves XIX and xx in group A (cf. Fig. 4 with Figs. 2 and 3). 
In group A the main nerve (xx) has eight times the strength of an ordi- 
nary spinal nerve. In group B the main nerve (xx1) is but six times as 
strong as this unit. In group A nerve xix has an average strength 
$ to 3 that of nerve xx; though in some cases (30%) their strengths are 
equal, in others xIx may have only ł the strength of xx. In group B, on 
the other hand, in two cases only is xx weaker than xxr; in three cases 
their strength is equal, and in two cases xx is the stronger, i. e. in 70% 
of the cases xx equals or is stronger than xxr. The average strength 
of xx as compared with xxi is as 1.1 to 1 in group B, whereas in group 
A the ratio between the corresponding elements is about 0.6 to 1. This 
shows that in group B the main nerve (xx1) tends to be weaker, and the 
one next anterior to it (xx) to be stronger than the corresponding nerves 
(xx and xix) in group A. This, together with the weakness and incon- 
stancy of XXII in group B, and the greater strength of xix with its in- 
creased tendency in this group to enter the plexus, is evidence that in 
group B the strength-centre of the plexus has not travelled caudad through 
an entire segment as compared with group A, but holds a position inter- 
mediate between such an one and that of group A. 
If we adopt as an explanation of the above facts and conclusion the 
interpolation of a presacral segment, we shall have to supplement such 
an explanation by assuming a subsequent movement cephalad of the 
plexus as a whole, to account for the ascertained intermediate position 
of the strength-centre of the plexus. 
From the evidence of the heemal arch, and of the topography (8 type) 
of the plexus, — which I shall immediately discuss, — it seems more 
reasonable to consider this intermediate position of the plexus the result 
of what may be called the migration caudad of the locus of the strength- 
centre. The correlation of the position of this locus with that of the 
girdle I shall discuss further on. 
There is in group B a persistence of the £ type of topography. This 
at first may seem to contradict the conclusion that we have an interme- 
diate position of the locus of the strength-centre, for with an interme- 
diate position of the plexus arising from migration caudad we should 
expect traces of reversion toward the more anterior position, such as is 
indicated by the tendency cephalad in the a type. However, there are 
reasons for expecting the B type here. (1) In such posterior migra- 
tion as is shown in group B, there must also be a tendency — whatever 
has been the stimulus to cause the migration of the locus of the whole 
plexus — for a movement caudad among the parts of the plexus, since 
