TRANSPLANTING NEURAL TUBE OF AMBLYSTOMA 183 



is smooth and rounded without the shghtest trace of outgrowth 

 forward. In fact, the appearance of sections points to what 

 amounts to a repulsive effect, or negative chemotaxis, between 

 the posterior stump and transplant, or at least the complete 

 absence of any attraction between the two regions. The sig- 

 nificance, therefore, of the time interval between the formation 

 of the anterior and posterior connections would seem to be that 

 it represents the time required for the descending fibers from the 

 anterior stump to make their way through the transplant; which 

 they do in the direction of the original anteroposterior axis of 

 the transplant. After traversing the transplant a short distance, 

 the descending fibers again change their direction and leave the 

 transplant to bridge the gap between it and the posterior stump. 

 All evidence of repulsion between the two regions would then 

 disappear just as soon as the descending fibers have opportunity 

 to penetrate the posterior stump, which they do, presumably, 

 because the posterior stump is territory that they traverse in 

 normal development. 



Incidentally, if the above interpretation be the correct one, it 

 indicates that the first long tracts connecting the brain and cord 

 are motor. After the motor connections are established through 

 the transplant, there is everj^ reason to beheve that sensory tracts 

 develop, with the result that there is restored a condition that is 

 almost normal, but not completely so, for the period during which 

 the embryos were under observation. To what extent the res- 

 toration approaches normal conditions remains to be seen. 



Hooker ('17), in his study of the effect of reversing a section 

 of the cord in the frog, found a marked tendency on the part of 

 the nerve fibers to avoid entering the opposite wound surfaces. 

 The conditions of his experiments excluded any opportunity for 

 the dissipation of this state of mutual repulsion, with the result 

 that nerve connections failed to develop between the ends of the 

 reversed cord and the stumps. In my experiments the conditions 

 are different, and the ' avoiding reaction' is more hkely a phenom- 

 enon of the posterior stump only — the transplant being neutral, 

 so to speak. The question as to why ascending fibers do not 

 arise from the posterior stump would seem to have its answer 



