70 FRANCIS DARWIN. 



clearly visible increments, and may ultimately attain the 

 length of nearly one millimeter. The filaments appear to 

 pass through the substance of the external cell- wall of the 

 glands, as no apertures to allow of their passage have been 

 observed. 



Under normal circumstances the filament presents the 

 appearance of a delicate and elongated thread slightly club- 

 bed at its free end, and animated by the perpetual tremble 

 of Brownian movement. The distal end of the filament is 

 often attached to the gland, thus forming a loop. Extremely 

 delicate filaments of great length are often seen entangled in 

 elaborate and complex knots, or several filaments may be 

 seen issuing from a single gland. 



The substance of which the filaments are composed is 

 gelatinous, transparent, highly refracting, and devoid of 

 granules. It is stained by tincture of alkanet, and not 

 blackened by osmic acid, and coloured yellow by iodine. 

 These reactions, when combined with results of various 

 physiological tests, show that the filaments contain resinous 

 matter in some way suspended in protoplasm. 



The most remarkable point in the behaviour of the fila- 

 ments is their power of violently contracting. Contraction 

 usually begins at a number of nearly equidistant points, 

 situated close together near the free end of the filament. 

 The curious beading thus produced spreads rapidly down the 

 filament, which ultimately runs violently together into a 

 ball seated on the top of the gland. In other cases con- 

 traction takes place without any previous appearance of 

 beading. 



Filaments frequently break loose but retain their vitality, 

 and are still capable of contraction although separated from 

 their parent glands ; and this observation is of importance, 

 as proving that the movements of the filaments are not 

 governed by forces residing within the glands, but that the 

 filaments are composed of an essentially contractile substance. 



The contraction of the filaments is produced by the fol- 

 lowing causes : 



Dilute acids (from 1 to i per cent.) : — Sulphuric, hydro- 

 chloric, acetic, citric, and osmic acids. 



Dilute alkaline solutions {\ to ^ per cent.) : — Carbonates 

 of ammonia, sodium, potassium. 



Solutions of gold-chloride ^ per cent., silver nitrate \ per 

 cent., sulphate of quinine -p-g- per cent., citrate of strychnia 

 (about) f per cent., camphor -^V per cent., the poison of the 

 cobra (about) \ per cent., iodine \ per cent. 



Glycerine. 



