No. 2.] THE STUDY OF VATA TION. ATT 
general proportions of the respective vertebral regions would 
thus be altered, and a tendency would result for the XIX 
segment to be laid down in a relatively more anterior position. 
The sacrum, in other words, would tend towards forward 
homeceosis. The two specimens just mentioned should, then, 
present forward homeeosis, and both do. 
On Baur’s ground, also, both specimens should show forward 
homeeosis, but on his ground that portion of the body lying in 
front of the normal XIX segment should show, in addition, 
an increase in absolute length proportional to the numerical 
increase of the vertebra. In the specimens at hand such a 
proportional increase in the length of the trunk region does 
not take place. Specimen No. 46, though possessed of an 
additional pre-sacral vertebra, has its sacrum, so far as the 
general proportions of the body are concerned, in the normal 
position, z.¢., on abscissa 33%, as will be observed by reference 
to Plate C; while No. 61, far from having a longer pre-sacral 
region, has this region remarkably short ; indeed, in the one 
hundred specimens of which proportional measurements have 
been taken, only two show an equal amount of pre-sacral com- 
pression. 
Again, according to the theory of intercalation, if the first 
hzemal arch occurs normally on the XXIII vertebra, four 
joints behind the sacrum, on the occasion of the introduction 
of an additional pre-sacral vertebra, the pelvis should be simply 
forced backward with all its chattels. This, however, does not 
occur, —the hzmal arch does not change its position from 
the X XIII vertebra. 
On the theory of regional compression and expansion, which 
we have advanced, the hemal arch, a function of the axial 
rather than of the appendicular areas, should tend in forwardly 
homoceotic specimens to approach the pelvic arch, z.e., in homee- 
otic specimens the number of vertebrz intervening between 
the sacrum and the first hamal arch should tend towards 
reduction. This is what actually occurs. (See Section VII, 
P. 473.) 
The assumption that the embryonic pelvic girdle travels 
backward and forward over a fixed vertebral column has been 
