WAITE: PLEXI OF NECTURUS. 85 
I have shown that no intercalation or excalation is evident in the 
prebrachial region of Necturus, and since serial variation tends to be at 
one end of the series, it is more to be expected in the prebrachial region 
than in the postbrachial-presacral region. Addition or loss at the caudal 
md of the series may be disregarded, as this could not directly affect 
the number of presacral vertebra, although it might be correlated with 
an abnormal position of the sacrum, both being an indication of general 
instability in the individual. 
While the idea of variation in the number and position of centres of 
metamerism may be attractive on theoretic grounds, it has no observa- 
tional evidence to support it, and, further, is insufficient to account for 
unsymmetrical sacra and supernumerary sacral ribs. On the other 
hand, intercalation or excalation has not been shown to exist as a mor- 
phological process, the supposed evidence for it being more naturally 
interpreted as due to pathological conditions. Since these two explana- 
tions seem improbable, we must look for some other cause of the varia- 
tions of presacral distance which we find. 
The common expression of sliding or shoving of the sacrum upon the 
column is open to criticism. It is contrary to ideas of metameric unity 
that the girdle should begin its development in one segment, and later 
in ontogeny migrate to the next. Bolk (94, p. 267) has found in Homo 
during ontogeny an actual migration cephalad on the part of the Anlage 
of the pelvic girdle ; but such migration is confined within narrow limits, 
not having extended over an entire segment, and thus the Anlage has 
not passed through a myotome. A slight migration of the musculature 
in relation to the skeleton is also known, but none of these slight 
changes are sufficient to account for sudden displacement of the girdle 
through an entire segment. Bumpus (’97, p. 464) has advanced a mod- 
ification of this idea to account for the greater frequency of displace- 
ment caudad. Since the transverse processes to which the sacral ribs 
are attached lie nearer the caudad limit of the vertebra, he considers it 
“more probable that variations occurring in the course of ontogenetic 
development will fall on the side of nearer proximity,” i. e. into the 
next segment caudad, and, having once invaded the territory of this new 
segment, the ribs will be adjusted to the proper position within that 
segment near the posterior limit. The individuality of segments in the 
skeleton is very sharp, and any explanation which involves a migration 
of an organ located entirely in one segment, so that it comes to lie 
wholly in another segment, assumes a process which is not ‘only un- 
proved, but seems to me highly improbable. Bumpus accounts for 
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