20 H. V. NEAL 



may be regarded as variations of a theme rather than as original 

 themes. The brothers Hertwig, for example, on the basis of 

 evidence derived from the study of the development of nerves 

 in medusae, suggested ('78) the hypothesis that the terminal 

 organ, primarily separated from the nerve center, becomes sec- 

 ondarily connected with it by means of a simple protoplasmic 

 path which is later differentiated into the actual nerve. The 

 connection between nerve center and terminal-organ is formed 

 secondarily, is at first protoplasmic and not nervous, and is the 

 product of indifferent cells and not those which form the nerve 

 fibers. Such protoplasmic connections become the paths along 

 which the nerves develop and are established when nerve center 

 and terminal-organ are in close proximity. This hypothesis, in 

 common with that of Hensen, lays stress upon the protoplasmic 

 paths within which the definite nerve fibers are differentiated, 

 but differs from the Hensen hypothesis in assuming the second- 

 ary connection between center and terminal-organ. The hy- 

 pothesis, however, as Braus ('05, p. 472) suggests, may not be 

 sharply distinguished from the cell-chain hypothesis which has 

 been rendered untenable or unnecessary by Braus' experiments 

 upon amphibian larvae. Braus admits, however, the possibility 

 of the formation of protoplasmic bridges before the appearance 

 of the neurilemma cells. But, since it has not yet been proved 

 that the protoplasmic bridges are the products of indifferent 

 cells — in other words, that they are not the product of the same 

 neuroblasts which produce the neurofibrillae — the hypothesis re- 

 mains little more than a speculation, and the phenomena upon 

 which it seems to rely are equally well interpreted by the process 

 theory. 



Other investigators, such as Apathy ('97), Joris ('041, Pighini 

 ('04, '05), Besta ('04), London and Pesker ('06), Brock and 

 Gieriich ('06, '07), and Held ('09) have presented some diver- 

 gent opinions, but no wholly distinct point of view. The pecul- 

 iar views of Apathy possibly deserve special recognition since 

 they have aroused so great an interest. According to Apathy 

 ('97) the embryonic ganglion cells acquire their fibrillar struc- 

 ture, not by a process of cell differentiation, but secondarily 



