72 MONTROSE T. BURROWS 



NERVE FIBRES 



Growth of the nerve cells is evidenced by filaments of various 

 sizes, which appear along the border of a piece of neural tube and 

 which grow out along a wav^^ course in the transparent clot. These 

 filaments vary in size from very fine threads to coarse cord-like 

 strands. The slender filaments are composed of a hyaline homo- 

 geneous protoplasm, while in the coarser bundles the homogene- 

 ous character is altered by the appearance of delicate, longitudi- 

 nal striations. The latter bundles break up into many fine fila- 

 mentous branches, either at their ends or along their periphery. 

 At the end of each of these growing filaments and branches is 

 the characteristic thickened amoeboid swelling as described by 

 Harrison. This is an oval or round swelling of the filament from 

 which protrude many actively moving delicate pseudopodia. The 

 growth of a fibre consists in the great prolongation and enlarge- 

 ment of one of these pseudopodia with a gradual moving out- 

 ward of the end knob along the pseudopod. The growth may be 

 so rapid that the end knob may entirely disappear, to reappear 

 farther out along the new grown part. 



The growth of nerve fibers in any culture is always limited to a 

 short period, from forty-eight to seventy-two hours. During this 

 time they may grow very rapidly, a micron to a micron and a half 

 in a minute, and reach a length of from one to two millimeters. 

 Othei- nerves remaining active for a long period may never reach 

 any considerable length. The activity of such fibres is noted at 

 the amoeboid end and consists in a constant retraction and new 

 formation of pseudopodia. All observations on the movement of 

 the growing fibre suggest an active force within it causing its 

 extension into the medium. 



True degeneration of the nerves has occurred in only two 

 per cent of the cases. This appeared in the form of nodosities 

 with some fragmentation after four days of growth. The 

 moie common end of the life history of a nerve fibre is re- 

 traction. This phenomenon is noted a few hours after a complete 

 cessation of movement. It appears first as a pronounced length- 

 ening of the thickened end accompanied by marked shortening 



