74 MONTEOSE T. BURROWS 



of the fibre. The shortenmg is generally rapid, greatly exceeding in 

 rapidity the greatest rate of extension. The changes in the nerves 

 are mainly at the end. Here there is periodic thickening, followed 

 by a slow reduction in size until the entire nerve has retracted 

 into the tissue in a manner similar to the retraction of the psuedo- 

 podium of an amoeba. These phenomena of extension and re- 

 traction may go on alternately in the same fibre. There is fre- 

 quently extension of the fibre for a considerable period of time, 

 followed by a period of quiescence and retraction. The retraction 

 is checked after a time and growth again proceeds in a different 

 direction for a while when the process is again repeated. This 

 phenemenon is shown to some extent in fig. 1. 



That these fibres are identical with the nerve fibres of the em 

 bryo seemed without question from their general morphology and 

 their origin from nerve tissues only. To complete, however, the 

 general histological methods of identification, the reaction of the 

 fibres to various stains, haematoxylin and Congo red, Cajal re- 

 duced silver method and Held's molybdic haematoxylin, was de- 

 termined. With both the Cajal and the Held methods the nerve 

 fibres take the characteristics color. The Held stain, however, 

 has been used for the preparations figured and described below 

 on account of the greater contrast shown in the experiments 

 stained by this method. The fibrin background has stained 

 deeply with the reduced silver method and obscured consider- 

 ably the slightly deeper staining nerve fibres. 



The individual neuro-fibrillae are shown clearly in the stained 

 preparations. The larger bundles appear as twisted rope-like 

 strands or flat layers of delicate fibrillae (fig. 4). The very fine 

 nerves stain more homogeneously, and often only a single fibril 

 can be distinguished throughout their entire course. The end 

 bulbs appear as faintly stained enlargements at the end of 

 the axis cylinders (fig. 5). 



The end bulbs and their pseudopodia stain irregularly, show- 

 ing one or more dark staining bands which pass from the nerve 

 fibre to the different pseudopodia (fig. 9). This dark-staining 

 material ma}^ often be broken and appear as dark-staining globules, 

 lying in the main mass of the bulb (fig. 5) . In some cases the fibres 



