HISTOLOGICAL METHODS. 



231 



If the preparation is to be kept permanently, the acid must all be 

 washed out, and dilute glycerine run under the cover glass. The prepa- 

 ration should then be sealed with Canada balsam or some other cement, 

 but previously all trace of glycerine must be removed from the slide and 

 upper surface of the cover glass. It is generally best to gently wipe the 

 edge of the cover glass with a small brush moistened with alcohol before 

 applying the cement. 



If the spore mother cells are still quite young, we shall find the nucleus 

 (Fig. 127, A, n) comparatively small, and presenting a granular appear- 

 ance when strongly 

 magnified. These 

 granules, which 

 appear isolated, are 

 really parts of fila- 

 ments or segments, 

 which are closely 

 twisted together, 

 but scarcely visible 

 in the resting nu- 

 cleus. On one side 

 of the nucleus may 

 usually be seen a 

 large nucleolus 

 (called here, from 

 its lateral position, 

 paranucleus), and 

 the whole nucleus is sharply separated from the surrounding protoplasm 

 by a thin but evident membrane. 



The first indication of the approaching division of the nucleus is an 

 evident increase in size (B), and at the same time the colored granules 

 become larger, and show more clearly that they are in lines indicating the 

 form of the segments. These granules next become more or less conflu- 

 ent, and the segments become very evident, appearing as deeply stained, 

 much-twisted threads filling the nuclear cavity (Fig. 127, (7), and about 

 this time the nucleolus disappears. 



The next step is the disappearance of the nuclear membrane so that 

 the segments lie apparently free in the protoplasm of the cell. They 

 arrange themselves in a flat plate in the middle of the cell, this plate 

 appearing, when seen from the side, as a band running across the middle 

 of the cell. (Fig. 127, D, shows this plate as seen from the side, E seen 

 from above.) 



FIG. 127. A, pollen mother cell of the wild onion. ?., 

 nucleus. B-F, early stages in the division of the 

 nucleus, par. nucleolus ; acetic acid, gentian violet, 

 x 350. 



