SPIROGYRA 445 



difficulty is however that the process normally takes 

 place at night, beginning about 10 to 12 P.M. Stras- 

 burger, who has made this a subject of careful study, 

 recommends that the Spirogyra be placed in a flat 

 plate upon a block of ice during the night : if on the 

 following morning the plants be exposed to a higher 

 temperature, the cell-division which had been pre- 

 viously retarded will begin, and the successive stages 

 may be followed. 



Though the main points in the process of division may be 

 observed in fresh material, the minute details will be better seen 

 in material which has been fixed in 1 per cent, chromic acid for 

 about four hours, or in picric acid : after washing carefully, the 

 material may be stained with carmine or hsematoxylin, and 

 mounted in very weak glycerine. 



The following points in the process should be specially 

 noted 



1. The disappearance of the nucleolus. 



2. The formation of the striated nuclear spindle and 

 of the equatorial nuclear disk. 



3. Division of the chromatin which constitutes the 

 disk, and the collection of the two halves at the poles 

 of the spindle as the new nuclei, which are still con- 

 nected by fine threads. 



4. The subsequent dilatation of the spindle, the 

 threads becoming more curved, while from it new con- 

 necting threads pass to the peripheral protoplasm. 



5. Meanwhile microsomata collect at the equator 

 of the dividing cell. 



6. Note also the involution of the chromatophores 

 and their subsequent division. 



