METAMEIUC SEGMENTATION AND liO^^rOLOGY. 
233 
is it to explain the next set of cases, in wliicli the plexus is 
modified in extent or position without an accompanying 
change in the vertebral column. For instance, in the rabbit 
the luinbo-sacral nerve-plexus may in some individuals include 
numbers 25-30 and in others numbers 24-30, while the first 
sacral vertebra is the twenty-seventh in both cases. Here 
a nerve seems to have been intercalated in the middle of 
the plexus, since the crural nerve (coming chiefly from 26 
and 27 in the first case) appears to have moved one segment 
forwards, the ischiadic remaining behind in its original 
position. 
To account for these changes, von Jhering has to make 
anotherassumption : itis that the nervous systeinbeing derived 
from the epiblast and the skeleton and muscles from the 
mesoblast, they can vary independently, and that a segment 
having been intercalated or excalatedin the series of uerves,a 
re-arrangement takes place so that the plexus fits again on to 
the unaltered vertebral column in a new position. The 
nerves slip a cog, so to speak, and catch on in a new place. 
The whole theory becomes very artificial. By the dropping 
out now of nervous segments, and now of skeletal segments, 
and again of complete segments comprising both elements, it 
seems possible to explain almost any conceivable variation in 
the disposition of these parts. But what evidence is there to 
support these assumptions ? none whatever. 
If the difficulties are great in the way of explaining 
the varying position of the limbs in these simple cases, they 
are formidable indeed when applied to those forms in wdiicli 
the differences are more pronounced, as, for instance, in the 
fishes. The smelt, Osmerus eperlanus, has a pectoral fin 
supplied by the spinal nerves 1-4, and a pelvic supplied by 
nerves 18-29 (Hammarsten, 16), while in the whiting, G-adus 
merlangus, the nerves 1-4, supply the pectoral fin, and 
nerves 5-6 the pelvic (Diagram 3). Again, in Scymnus the 
pectorals are supplied from nerves 2-13, and the pelvics 
from nerves 23-35 j in Torpedo nerves 4-30 supply the 
pectoral, and nerves 31-42 the pelvics (Diagram 2). If tlie 
