6 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



is difficult to avoid when these lie in the immediate neigh- 

 bourhood of other nerve trunks. By rendering these 

 latter inexcitable by reason of their degeneration, the 

 source of fallacy can be completely set aside. 



In the above, great stress is obviously laid by implication 

 upon the distribution of the nerves as a key to the mor- 

 phological, especially to the segmental or metameric 

 architecture of the limb. Why are the nerve-fibres such 

 extremely important indices in this problem ? The nerve- 

 cells in the mammal, at least, are peculiar in not evincing 

 reproductive activity after a very early period in the 

 individual's life. They are early differentiated, and are like a 

 committee formed without power to add their numbers. 

 They are laid down when the metamerism of the relatively 

 simply-shaped young individual is more easily traced than 

 in the variously metaphorphosed adult. Thus laid down 

 the nerve-cell keeps in touch with the remainder of that 

 segment to which it belongs in the body by virtue of the 

 enormous power of individual growth of its cell-branches. 

 It does not, as in the young muscle-cell, keep pace with the 

 extension of the body segment in any one direction by means 

 of a repeated fission of nucleus and reproduction, and the 

 forming of a chain of muscle-cells, more or less nutritively 

 independent. To trace such a line of units, although they 

 are closely connected by blood-relationship, is impossible 

 with certainty by any known method. But the nerve- 

 cells remains securely anchored in their original segmental 

 positions, both inside the spinal cord and outside it, in 

 the row of segmentally-arranged spinal ganglia. Growing 

 out from the nerve-cells their nerve-fibres wander dis- 

 tally into skin and muscles, and the main difficulty in 

 tracing the actual limits of the metameres in these last, 

 is that the elements of the adjacent metameres certainly 

 considerably intermingle. So it is also with the ramifica- 



