186 H. L. WIEMAN 



General conclusions may be summarized as follows: 

 I. When a small section of the neural tube of Amblystoma, at 

 the stage of the closed neural folds, is removed at the level of 

 the second to fourth somites, together with portions of adjacent 

 somites, and reimplanted at right angles to its normal position : 



1. The transplanted tube continues its development and re- 

 tains its original polarity. 



2. The growing brain, pushing the anterior stump back against 

 the anterior side of the transplanted tube, may be a factor in 

 forming a union at the point of contact. 



3. Nerve fibers grow back from the anterior stump into the 

 transplant near its original anterior end. 



4. The posterior stump becomes club-shaped, but shows no 

 initial tendency to send fibers forward into the transplant. 



5. A connection between the transplant and the posterior 

 stump is brought about by the continued growth backward of 

 fibers from the anterior stump through the transplant. 



6. Ascending fibers then grow forward from the posterior 

 stump. 



7. The transplant eventually becomes absorbed in the recon- 

 structed neural tube, but the appearance of sections shows that 

 the cells of the transplant do not lose their original polarity as 

 late as thirty to forty days after operation. 



II. ^^Tlen the operation is performed at a stage just before the 

 larva becomes sensitive, the process is the same, but the union 

 is formed in a shorter time after operation. 



III. When the operation is performed at the earlier stages, 

 but in the region of the fifth and sixth somites, or farther back: 



1. A longer time is required for the formation of nervous 

 connections. 



2. In series of twelve operations no case of complete con- 

 nections between the stumps occurred in thirty days. 



3. In one case at least the transplant was disintegrated at the 

 end of thirty days. 



