No. 2.] THE STUDY OF VARIATION. 465 
“normal” vertebral segment, vzz., the XIX. We assume, 
moreover, that when the relative position of the appendage is 
once determined in the embryo of Necturus it is determined 
for life ; since there is no evidence to show that there is a 
“migration’’ of the parts of the pelvic region after the first 
rudiments are laiddown. Rosenberg (’76), however, claims a true 
pelvic migration for Homo, involving an emancipation of sacral 
elements for the production of coccygeal vertebrae, and Credner 
(86) claims a distal shifting of the pelvic arch in the fossil 
amphibian Branchiosaurus. There are grounds, moreover, as 
will be shown in Section V, for the belief that the very process 
of local differentiation of the embryonic cells, for the final pro- 
duction of the appendage peripheral to the vertebral axis, occurs 
in a position in no respect dependent upon the position of any 
one vertebral segment, but dependent rather upon the general 
proportions of the embryo asa whole. The determination of 
the loci of the successive vertebrze and their early differentia- 
tion exerts no determining influence on the position of the 
appendages. But the Anlage of the appendage once deter- 
mined, influence from it will direct the growth of proper sacral 
elements in the nearest vertebral segment, be it the XVIII, 
DEE On, XX. 
It should furthermore be noted that when the Anlage of 
the appendage does not fall within the territory of the normal 
metamere (XIX), its vertebra, which has presumably been pro- 
ducing sacral ribs in two-thirds of the specimens since some 
remote geological period, does not show the slightest sign of 
sacral differentiation, but is exactly like the other neighboring 
trunk vertebrz. 
Section IV. 
Can an explanation be given for the frequent occurrence of 
oblique or unsymmetrical sacra ? 
In eight per cent of the specimens examined the sacrum is 
not a single vertebra, but is composed of two halves, each 
belonging to different metameres (Plate A, Spec. 62). The 
legs, moreover, in these specimens, leave the body at points 
not directly opposite. 
