WAITE : PLEXI OF NECTUEUS. 85 



I have shown that uo 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 tlie j^rebrachial region 

 than in the postbrachial-presacral region. Addition or loss at the caudal 

 end of the series may be disregarded, as this conld not directly affect 

 the number of presacral vertebree, 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 



VOL. XXXI. NO. 4. 2 



