METAMERIC SEGMENTATTOX AXD HOMOLOGY. 
229 
Stellio ; the twenty-seventh and twenty-eighth in Ascala- 
botes ; the twenty-ninth and thirtieth in Lacerta ; the sixty- 
fifth and sixty-sixth in Seps. In birds also we find a remark- 
able inconstancy in the relative position of the limbs [for an 
admirable and detailed study of this cpiestion see Furbringer’s 
great monograph, ( 10 )]. To take only three examples: in 
Cypselns the brachial plexus extends over segments 10-14, 
and the pelvic over segments 20-27 ; in Podiceps the brachial 
plexus occupies segments 15-18, and the pelvic segments 26— 
35; while Struthio has a brachial plexus in segments 17-21, 
and a pelvic plexus in segments 29-38. (see Diagram 1). 
Even the Mammalia show considerable variation in the 
position of the pelvic limb, although that of the pectoral limb 
remains remarkably constant. The lurnbo-sacral plexus of the 
rat includes* the lumbar nerves 2-6, of the guinea-pig the 
lumbar nerves 3-6 and first sacral, of the gnu the lumbar 
nerves 4-6 and first and second sacral [Paterson ( 18 )]. 
Xo less conspicuous is the shifting of the paired fins in 
fishes, as will be easily seen on consulting Diagrams 2 
and 3. 
In the foregoing account the genetic relation of the limbs 
to the segnnents has been determined by the nerve-supply in 
the adult. This is amply justified by a study of the development, 
which shows that any change of position during* ontogeny, 
caused by the shifting of the limb-rudiment from its place of 
origin, is insignificant. The limb is not formed on one set of 
segments, and subsequently transferred to another region and 
supplied b}” new nerv'es. On the contrary, limbs arise in the 
embryo in that region which they occupy in the adult. As a 
rule several segments contribute to the formation of each 
limb, the skeletal tissue musculature and nerves being derived 
from these segments. AVhen, as in many fish, the radial 
muscles of the paired fins are formed from buds of the myo- 
tonies, their exact relation to the segments can be accurately 
made out. Concentration,” or the relative narrowing of the 
base of attachment of the fin, may introduce secondary 
modifications, which do not, however, affect the position of 
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