Exprrntients in Transplantnig Lntihs 2/5 



Coming from a physiologist, the especial virtue of the theory, as 

 might be expected, is physiological; it places the genesis of the 

 permanent nerve paths upon the basis of functional adaptation; 

 of all the numerous undifferentiated protoplasmic connections 

 existing in the embryonic body, it is only those which function in 

 conducting impulses that persist as permanent nerves; the remain- 

 der atrophy from disease. In refutation of this hypothesis it may 

 be pointed out, however, that the functional activity of a nerve has 

 no appreciable influence at least upon its early development. 

 Amphibian embryos reared in a solution of acetone chloroform^^ 

 acquire a perfectly formed nervous system, and one capable of 

 normal functional activity, though during the whole period of 

 their development up to the stage when the yolk is entirely 

 absorbed, at which time the peripheral nerves are all well differ- 

 entiated, no functional activity of the nervous system is mani- 

 fested. Furthermore, in the transplantation experiments just 

 referred to, a number of nerves were formed to which no conceiv- 

 able function could be assigned, as, for instance, the funicular 

 fibers which, after the removal of the medullary cord of the trunk, 

 extend out from the brain and lose themselves in the mesenchyme. 

 It is precisely in this connection that the experiments in trans- 

 planting nerveless limbs are of great significance. A nerveless 

 limb is taken from an organism that has undergone the greater 

 part of its development after having been deprived of its spinal 

 cord. As a consequence, no nerves are developed in the trunk 

 region, and there is no evidence of nervous activity there, although 

 in a normal individual during the same period, all of the important 

 nerves, including those running to the extremities are visibly dif- 

 ferentiated and for the most part are functioning. It cannot be 

 supposed that the pre-nervous protoplasmic bridges, postulated by 

 Hensen's theory, would be able to survive this long period of dis- 

 use, for, as experiments show, even visibly differentiated nerve 

 fibers degenerate very rapidly after removal of their centers, often 

 disappearing without leaving a trace in a much shorter time than 

 that during which the nerveless individuals in question have been 



'^Harrison '04. 



