Scientific Proceedings. 



143 



It has not yet been found possible to make permanent speci- 

 mens which show the isolated nerve fibers completely intact. The 

 structures are so delicate that the mere immersion in the preserv- 

 ing fluid is sufficient to cause violent tearing and this very fre- 

 quently results in the tearing away of the tissue in its entirety 

 from the clot. Nevertheless, sections have been cut of some of 

 the specimens and nerves have been traced from the walls of the 

 medullary tube, but they were in all cases broken off short. 



In view of this difficulty an effort, which resulted successfully, 

 was made to obtain permanent specimens in a somewhat different 

 way. A piece of medullary cord about four or five segments 

 long was excised from an embryo and this was replaced by a 

 cylindrical clot of proper length and caliber, which was obtained 

 by allowing blood or lymph of an adult frog to clot in a capillary 

 tube. No difficulty was experienced in healing the clot into the 

 embryo in proper position. After two, three or four days the 

 specimens were preserved and examined in serial sections. It was 

 found that the funicular fibers from the brain and anterior part of 

 the cord, consisting of naked axones without sheath cells, had 

 grown for a considerable distance into the clot. 



These observations show beyond question that the nerve fiber 

 develops by the outflowing of protoplasm from the central cells. 

 This protoplasm retains its amoeboid activity at its distal end, the 

 result being that it is drawn out into a long thread which becomes 

 the axis cylinder. No other cells or living structures take part in 

 this process. The development of the nerve fiber is thus brought 

 about by means of one of the very primitive properties of living 

 protoplasm, ambceoid movement, which, though probably com- 

 mon to some extent to all the cells of the embryo, is especially 

 accentuated in the nerve cells at this period of development. 



The possibility becomes apparent of applying the above method 

 to the study of the influences which act upon a growing nerve. 

 While at present it seems certain that the mere outgrowth of the 

 fibers is largely independent of external stimuli, it is of course 

 probable that in the body of the embryo there are many influ- 

 ences which guide the moving end and bring about contact with 

 the proper end structure. The method here employed may be of 

 value in analyzing these factors. 



