MITOSIS IN POLLEN MOTHER-CELLS. 25 



The young weft-like nuclear membrane encloses a cavity containing 

 the chromatin and little or no other staining material. With further 

 development the kinoplasmic weft is transformed into the typical 

 nuclear membrane, appearing in section as a sharp line, and the 

 daughter spirem becomes loose and open. En the mature daughter 

 nucleus the spirem is continuous and of a tolerably uniform thickness. 

 In some cases it is rather regular, consisting of long turns arranged in 

 the form of a wreath (Fig. 10, K), but in the majority of instances 

 the spirem is irregular, with long and short turns so disposed that its 

 course cannot be easily followed. This condition of the spirem is in 

 all probability due to the variously shaped chromosomes mentioned 

 in a preceding paragraph. 



THE NUCLEOLUS. 



In the resting nucleus and during the prophase, one or more nucle- 

 oli are present. These nucleoli take on a deep red or reddish purple 

 color with the Flemming triple stain. They sometimes present a uni- 

 form structure, but, as a rule, the larger nucleoli especially reveal one 

 or more vacuoles. As has been mentioned in a preceding paragraph, 

 the nucleolus very frequently lies within a spherical space which 

 appears in optical section as a colorless court about it. This phe- 

 nomenon is especially striking in vegetative cells of higher plants, 

 such as in root tips of Vicia faba and Zea mays. Experiments 

 seem to show that the colorless space surrounding the nucleolus 

 contains something more than a mere watery fluid which is extracted 

 in dehydration. By subjecting roots of Vicia, Zea and others to a 

 strong centrifugal force, the author (Mottier, '99) found that the 

 nucleolus together with its surrounding colorless court was thrown 

 out of the nucleus into the cytoplasm. The expelled nucleolus was 

 still surrounded by its colorless court a fact that seems to* show that 

 the colorless substance has a specific gravity much greater than other 

 constituents of the nucleolus, and that it may be provided with its own 

 membrane. This colorless substance may represent unorganized 

 nucleolar matter. 



Frequently before the nuclear membrane disappears a disorganiza- 

 tion begins by which the nucleolus is broken up into several smaller 

 nucleoli (Fig. 6, C). As the nuclear membrane fades away, and the 

 kinoplasmic fibers enter the nuclear cavity, numerous bodies are found 

 distributed in the cytoplasm which stain exactly as nucleoli, and there 

 is no doubt that these bodies represent nucleolar substance. These 

 extra-nuclear nucleoli were found to be more abundant in Lilium 

 martagon. In Lilium candidum there may be none, or only a few 



