78 BULLETIN : ilCSEUM OF COMPARATIVE ZOOLOGY. 



sacrum, to nerves xix aud xx in group A (cf. Fig. 4 with Figs. 2 and 3). 

 In o-roup A tlie main nerve (xx) lias eight times the strength of an ordi- 

 nary spinal nerve, in group B the main nerve (xxi) is but six times as 

 strong as this unit. In group A nerve xix has an average strength 

 I to I that of nerve xx ; though in some cases (30%) their strengths are 

 equal, in others xix may have only I the strength of xx. In group B, on 

 the other hand, in two cases only is xx weaker than xxi ; 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 xxi. 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 (xxi) 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 gi-eater 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 haemal arch, and of the topography (^ 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 ceplialad in the a type. However, there are 

 reasons for expecting the /3 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 raigi-ation of the locus of the whole 

 T^lexus — for a movement caudad among the parts of the plexus, since 



