274 



Ross Granville Harrison 



Experiments which have already been reported by Lewis and by 

 myselP are sufficient to decide in favor of the latter alternative. 

 Destroy the nerve centers of an embryo no nerves ever develop. 

 Transplant the centers containing ganglion cells to otherwise 

 sterile (nerveless) regions, nerves will develop radiating from them, 

 often following paths entirely unknown as nerve paths in the nor- 

 mal organism; in one case even the peritoneal cavity was bridged. 

 Alter in the most profound manner the path normally taken by 

 certain nerve fibers, at the same time leaving the nerve centers 

 intact, fibers will, nevertheless, develop in connection with the 

 center, following the normal direction of growth, though in strange 

 surroundings. Again, ^^ if the ganglion cells are removed after the 

 nerves are partially developed, further development ceases and 

 all traces of the nerve may entirely disappear; but on the other 

 hand, as is well known, if the nerve is removed leaving the center 

 intact, a new nerve is soon formed in its place. These facts show 

 that the first essential for the formation of the nerve fiber is the 

 ganglion cell. The only other condition, which, as far as known 

 at the present time, is necessary, is that there must be a surround- 

 ing medium in the form of living tissue. There is no evidence, 

 however, that any specifically formed or localized structures, essen- 

 tial to the formation of nerve fibers, are present within this 

 medium. ^^ 



Hensen's theory supposes that the protoplasmic bridges con- 

 necting the various cells of the embryonic body, play this part. 



^-Harrison '06; Lewis '06. 



^See p. 272. 



34Recently Held has maintained, in accordance with Hensen's theory, that the outgrowth of the 

 nerve fiber from the ganghon cell is only apparent and in reality is but a differentiation of preexisting 

 protoplasmic filaments found between the various organs in the embryo. "Entsteht also die Nerven- 

 leitung durch die Umwandlung von Plasmodesmen in Neurodesmen'' (op. cit., p. 188). A full dis- 

 cussion of this work will be deferred to a future communication dealing more especially with histogenesis. 

 It may, however, be pointed out here that it is by no means certain that the plasmodesms are not arte- 

 facts — products of coagulation (cf. Harrison, Arch. f. mikr. Anat., Bd. 57, p. 421, and v. Lenhossek, 

 Neurologisches Centralblatt, Bd. 26, p. 127). Nor can it be certain in consideration of the extreme 

 minuteness of the structures in question, whether the fine filaments of the nerve process actually run 

 inio the fine protoplasmic threads or along them. At least it is very siginficant to find that Ramon y 

 Cajal ('07), employing methods essentially similar to those of Held , nevertheless gives his full support to 

 the outgrowth view. 



