342 



Scientific Proceedings (123). 



163 (1910) 



The existence of an attracting stimulus in the development of 

 the central nervous system. 



By DAVENPORT HOOKER. 



[From the Department of Anatomy, School of Medicine, University 

 of Pittsburgh, Pittsburgh, Pa.] 



Studies on the spinal cord of amphibian embryos have demon- 

 strated the remarkable regenerative powers of the central nervous 

 system in these forms. The cord may be completely severed in 

 frog embryos and tadpoles up to the time of metamorphosis and 

 will restore anatomical and physiological continuity in a high 

 percentage of cases. In the earlier stages, the removal of a seg- 

 ment of the cord one or two myotomes in length is followed by 

 complete reestablishment of anatomical continuity in most cases. 

 Where restoration of anatomical continuity fails for various 

 reasons, the regenerated or developing nerve fibers have grown 

 toward the opposite cut end of the cord, sometimes by unusually 

 tortuous routes. 



A series of experiments on young embryos in which a segment 

 of the spinal cord was removed, rotated on its long axis and re- 

 implanted, gives further indication of the existence of an attrac- 

 ting stimulus as a factor in reestablishing the continuity of the 

 cord. 



These experiments indicate that it is possible to obtain re- 

 establishment of anatomical continuity between a portion of the 

 cord in its normal position and a segment which has been rotated 

 on its long axis through various arcs up to complete inversion (180 0 

 rotation). The restoration of physiological continuity has not 

 been conclusively demonstrated, though there is now much evi- 

 dence that it has occurred. Better healing is obtained when the 

 segment is rotated up to 90 0 than when it is rotated more. When 

 rotated 90 0 , there is a tendency for the regenerated tissue closing 

 the cord wound to rotate through the quarter turn of a spiral 

 to link up the like fiber tracts. This same tendency is present in 

 rotations of from 90 0 to 1 35 0 , but is not so clearly demonstrable. 

 The rotation of the regenerated area of the cord is caused by the 

 spiral course taken by the developing nerve fibers. 



