SPECIAL METHODS. 



159 



Fig. 33, ///), which are held together by the mucilage sepa- 

 rated with them and finally thrown off with them. 



This expulsion may be caused by various precipitates. 

 A very suitable one is chrome yellow (PbCrO 4 ), which is. 

 precipitated in the membranes by placing the algae, held 

 together by a thread, in a .25$ solution of potassium chro- 

 mate (K 2 CrO 4 ), then rinsing quickly in water, and finally 

 transferring them to a .25$ solution of lead acetate. To 

 obtain a heavy precipitate, this proceeding may be several 



FIG. 33. /, membrane and gelatinous sheath of Zygnema sp. (x 580). //, two Zygnema- 

 cells after deposit of chrome yellow (X 245). ///, membrane and gelatinous sheath of 

 Zygnema after deposit of chrome yellow (X 245). IV, the same of Pleurotceniunt Tra- 

 becula, after staining with fuchsin (X 950). V, the same of Staurastrum bicorne, after 

 staining with gentian violet (X 950). z, cell-membrane; g, gelatinous sheath. / to /// 

 after Klebs; /Fand V after Hauptfleisch. 



times repeated. But the expulsion takes place the more 

 rapidly the less chrome yellow is deposited, and may not be 

 completed for several days if the deposit be large. 



Finally, it may be remarked that this expulsion is not 

 directly dependent on the life of the protoplasm, and may 

 occur in dead individuals, under some circumstances. 



281. Klebs has also established the remarkable fact that 

 the gelatinous sheaths increase markedly in density in a 



