142 Society for Experimental Biology and Medicine. 



tended out into the surrounding lymph clot. It is these structures 

 which concern us at the present time. 



In the majority of cases the fibers were not observed until they 

 had almost completed their development, having been found 

 usually two, occasionally three and once or twice four days after 

 isolation of the tissue. They consist of an almost hyaline proto- 

 plasm, entirely devoid of the yolk granules, with which the cell- 

 bodies are gorged. Within this protoplasm there is no definiteness 

 of structure ; though a faint fibrillation may sometimes be observed 

 and faintly defined granules are discernible. The fibers are about 

 1.5-3/* thick an d their contours show here and there irregular 

 varicosities. The most remarkable feature of the fiber is its 

 enlarged end, from which extend numerous fine simple or branched 

 filaments. The end swelling bears a resemblance to certain rhizo- 

 pods and close observation reveals a continual change in form, 

 especially as regards the origin and branching of the filaments. 

 In fact the changes are so rapid that it is difficult to draw the details 

 accurately. It is clear we have before us a mass of protoplasm 

 undergoing amoeboid movements. If we examine sections of 

 young normal embryos shortly after the first nerves have devel- 

 oped, we find exactly similar structures at the end of the develop- 

 ing nerve fibers. This is especially so in the case of the fibers 

 which are connected with the giant cells described by Rohon and 

 Beard. 



Still more instructive are the cases in which the fiber is brought 

 under observation before it has completed its growth. Then it is 

 found that the end is very active and that its movement results in 

 the drawing out and lengthening of the fiber to which it is 

 attached. One fiber was observed to lengthen almost 20/4 in 25 

 minutes, another over 25 ft in 50 minutes. The longest fibers 

 observed were 0.2 mm. in length. 



When the placodal thickenings of the branchial region are 

 isolated, similar fibres are formed and in several of these cases 

 they have been seen to arise from individual cells. On the other 

 hand, other tissues of the embryo such as myotomes, yolk endo- 

 derm, notochord and indifferent ectoderm from the abdominal 

 region do not give rise to structures of this kind. There can 

 therefore be no doubt that we are dealing with a specific charac- 

 teristic of nervous tissue. 



