TRANSPLANTING NEURAL TUBE OF AMBLYSTOMA 185 



ends exists in cases where the stumps have failed to form unions 

 with the transplant (F series) . At the same time it seems doubtful 

 that the transplant can for an indefinite period maintain its 

 form and structure without establishing connections with the 

 stumps of the brain and cord. This is indicated by the results 

 of the F series, in which the operation was made at the level of 

 the fifth and sixth somites, and in which in only one case out of 

 four selected for sectioning was even a single connection developed 

 between the stumps and the transplant within a period of thirty 

 days. In the other three cases the transplant was separated 

 from the neural stumps by muscle tissue to such an extent that 

 nervous union seemed unlikely, if not impossible. In two of 

 these the transplant showed evidence of growth and develop- 

 ment, the original anterior end being larger, but in the third 

 (F3) the transplant was disintegrating. Presumably, the de- 

 generation of the transplant was due to its failure to form a 

 connection with the nervous system. 



As has already been mentioned, the results of the F series 

 supply additional evidence for the conclusion reached regarding 

 the manner in which the nervous stumps become reunited through 

 the transplant. In the T. R, T. L., and G series the operation 

 was performed in the region of the second to fourth somites, while 

 in the F series the operation was behind the fifth somite. If 

 the restoration of anatomical connection depends upon the down- 

 growth of descending fibers from the brain, then a longer time 

 would be required for such fibers to reach the site of operation 

 in the F series. The results obtained in the F series would seem, 

 then, to be due to the fact that the greater amount of time 

 required for the descending fibers to reach the level of operation 

 allowed transplanted somite to develop to such an extent on either 

 side of the transplanted tube as to form a barrier on either side 

 of it. That this barrier may be overcome on the anterior side 

 of the transplant, at least, in the course of thirty days is shown 

 by F4 (fig. 18), but whether or not similar connections would 

 have been made later on in all of the F series is of little moment 

 to the point under discussion. The primary reason for lack of 

 successful reunions in the F series would seem to be the more 

 posterior site of the operation as compared with the other series. 



