115 



cell by the advancing division-wall. If such filaments are now 

 more strongly magnified and the other cells carefully examined, 

 we shall probably find some in which the process is just begin- 

 ning. In these (PL CII. Fig. 2) the young division-wall appears 

 as a fine ring running around the middle of the cell, its edge di- 



rected toward the center of the cell-cavity, and pushing in the 

 protoplasmic sac, leaving an evident clear space between the lat- 

 ter and the side walls of the cell. In larger cells, especially, (Fig. 

 7) it is very evident that the chloroplasts and other granular con- 

 tents of the cell, do not occupy the extreme periphery of the pro- 

 toplasmic sac, but that this is composed of perfectly homogeneous 

 colorless plasma (Ectoplasm). 



If we wish to follow the process through in the same cell, it 

 is better to choose a rather small one, as the division does not 

 take so long as in the larger ones. At ordinary temperature 



* 



(20^^-25° C.) the whole process in such cells takes from four to 

 five hours. 



The division of the inner granular protoplasm is completed 

 some time before the division wall is closed, so that there is a 

 neck of perfectly colorless protoplasm connecting the two green 

 masses for some time before the connection between the daugh- 

 ter cells is completely severed, (Figs. 4, 5), and for some time 

 after the division is completed the chloroplasts and other 

 granules are contracted away from the division wall (Fig. 6). 



Similar to the process just described is what occurs in the dif- 

 ferent species of Spirogyra and similar forms. The great differ- 

 ence lies in the fact that cell-division is accompanied by the di- 

 vision of the single nucleus of the cell. 



Cell-division in the various species of Spirogyra takes place 

 normally at night, but it may be retarded by suddenly reducing 

 the temperature shortly before the time at which division would 

 ordinarily begin, and keeping the plants In this condition until 

 wanted. In cool weather, when the temperature sinks to near 

 the freezing point at night, it is generally sufficient to keep the 

 plants in a warm room during the day, and at about 10 p. m. to 

 place the vessel containing them out of doors for the night. If 

 they freeze slightly it will ordinarily do little harm. On bringing 

 the plants into the laboratory in the morning, some of the cells 

 will be pretty certain to begin division almost immediately. 



