26 CYTOMORPHOSIS 



development. The first question which we have to answer 

 is: Does a regressive development also occur? The pro- 

 gressive is well known to us and we know much about it. I 

 incline strongly to the opinion that it is the only kind of 

 development, but there are not lacking investigators who 

 have come to the belief that under certain conditions develop- 

 ment may be reversed. 



My point of view is determined in part by the fact that 

 it has been possible in cases where a regressive development 

 had been assumed to make sure by careful investigation that 

 opinion had been misled by appearances and that in reality 

 the development was progressive in these cases also. I may 

 mention three examples; first, the nerve fibers. If one cuts 

 through a nerve, the fibers in its peripheral part degenerate 

 quite rapidly. After several days, however, under favorable 

 conditions, newly formed nerve fibers appear in the peripheral 

 part. Many investigators have eagerly advanced the view 

 that these nerve fibers rise in their place and that they have 

 been newly formed in the degenerating nerve. More careful 

 research has made it certain that the newly formed fibers 

 have simply grown out upon the ends of the healthy fibers, 

 left in the central part of the nerve. If one cuts off the roots 

 of a tree, the roots which are separated from the trunk 

 decay; but if the tree is left one can find later in their place 

 living roots, which, however, have not arisen from the dying 

 roots, but have grown out from the central healthy parts. 

 The fundamental experiments of Harrison 8 make it sure 

 that nerve fibers in all cases are formed only in the way 

 mentioned. About the origin of nerve fibers there has been 

 a long controversy. My countryman, Harrison, has occupied 

 himself for several years with this question, and has supported 

 his conclusion by the most varied investigations. Four 



