MORPHOLOGY OF EYE MUSCLE NERVES 17 



lack of a fundamental histological method which effects a satisfactory 

 stain for embryonic as well as for adult nervous tissue. But the dis- 

 agreement is also partly due to the fact that it has been the concept 

 rather than the reality that has been observed. Finally the continued 

 conflict of opinion has been due to the fact that no comprehensive 

 observations have been brought together which make it. possible to 

 demonstrate in one and the same animal all of the essential stages in 

 the development of a definite nerve path, and to extend in a compara- 

 tive research over the chief groups of vertebrates, in order to find, in 

 the diversity of phenomena seen in different embryos, the same funda- 

 mental principle of nerve development. 



Held undertook this herculean task and has made a most 

 valuable contribution to our knowledge of the histogenesis of 

 the neurofibrillae, which, since the emphasis laid upon them by 

 Apathy, have come to be regarded as the essential elements of 

 the nerve fiber. In large measure Held's monograph is an at- 

 tempt to reconcile the divergent hypotheses of nerve histogenesis. 



Three chief theories of nerve histogenesis have been advanced. 

 Chronologically arranged these are: 



1. The theory advanced by von Baer in 1829 and later elab- 

 orated by Hensen ('64, '76, '08) of the primary connection of 

 the nervous center with its later innervated terminal-organ. The 

 fundamental idea underlying this von Baer-Hensen theory is 

 that of the necessity of a primitive connection between the nerve 

 and its terminal organ. In other words that it is impossible to 

 conceive of a nerve fiber finding its area of distribution or its 

 terminal-organ as the result of the free outgrowth of the nerve 

 cell. Hensen brought forward the idea of primary nerve paths, 

 which was founded on the actual observation that in the embryo 

 adjacent organs are connected with each other by means of 

 protoplasmic bridges. These were interpreted as remnants of 

 incomplete cell divisions and they were supposed to form the 

 primary nerve paths which later become transformed into the 

 definitive nerves or are used for nerve formation. How the 

 transformation occurs was unknown to Hensen, but he was con- 

 vinced that they did not develop from cell outgrowths. The 

 theory appeared to afford a satisfactory explanation of how nerves 

 find their way to their terminal-organ. Chiefly on the ground 

 of general principles, Gegenbaur ('98), Fiirbringer ('97), Kerr 



JOURNAL OF MORPHOLOGY, VOL. 25, XO. 1 



