26 Journal of Comparative Neurology. 



a case as that just mentioned, that this withdrawal is caused by 

 a swelling of the peripheral process just beneath the cuticula. 

 The part of the sensory hair withdrawn beneath the cuticula 

 often enters into and helps form this varicosity. Sometimes, 

 but more rarely, the apex of this sensory hair remains normal 

 but the base enlarges (Plate I, Fig. 25). In very many cases 

 the formation of a varicosity or the thickening of a considera- 

 ble portion of a peripheral process withdraws the sensory hair 

 and then the apex swells into a rounded knob (Plate I, Fig. 7). 

 If ammonium picrate or Bethe's fluid be run under the cover- 

 glass upon these sensory hairs while they are still in the normal 

 condition, the same changes take place in them, but more 

 quickly. 



It seems to me possible, after seeing the normal form in 

 living tissue, watching the actual formation of these artefacts, 

 and afterward observing their appearance in sections, that such 

 artefacts have been in the past described as normal structures. 

 It appears to me especially desirable that those cases in which 

 a peripheral process from a sensory cell is described as ending 

 in a little knob just beneath or in the cuticula, should be re- 

 investigated — if possible by means of living material. 



I consider I am amply justified in deciding that in Nereis 

 virens every vmicosity or beading found in the peripheral processes 

 of the bipolar nerve cells or any end-knobs found on their sensory 

 hairs are artefacts produced during post-mortem chafiges. Normally 

 these processes, or nerve-fibers, are cylindrical and almost uni- 

 form in diameter and the sensory hairs are cylindrical, bluntly 

 pointed rods which always project above the external surface of 

 the body. 



Allen ('94) made a study of the varicosities in the nerve 

 fibers of Crustacea and explains their formation as follows : 

 "Both the phenomena of beading and the formation of end- 

 swellings appear to be due to a simple physical cause, namely 

 the difference of surface tension between two fluids. A fluid 

 cylinder surrounded by some other fluid of different surface ten- 

 sion is in a state of unstable equilibrium and tends to break up 



