farr: cytokinesis of pollen-mother-cells 295 



nuclei. They rarely, If ever, run parallel or to the same region 

 on another membrane. Quite regularly these tufts of fibers seem 

 to be attached to the nuclear membrane just above the chromatic 

 mass resulting from the transformation of a chromosome. These 

 chromatic masses are practically all peripherally placed in the 

 nucleus. It has been several times suggested that the material 

 of the fibers is of nuclear origin, and this would indicate that the 

 chromatin may have something to do with their formation or 

 secretion from the nucleus. 



VII. The constriction furrows and nuclear migration 

 In the tetranucleate pollen-mother-cell we have a condition 

 which offers peculiar advantages for studies in nuclear migration, 

 especially inasmuch as when first formed the nuclei are closely 

 appressed to the plasma membrane (fig. 26), whereas after spore- 

 formation they lie in almost the exact center of the daughter cells. 

 (fig. 37). I have studied quite carefully the origin of the tetra- 

 hedral arrangement of the nuclei. It is by all means the pre- 

 vailing arrangement in Nicotiana. In the examination of prac- 

 tically all of the hundreds of mother-cells in a single stamen, only 

 four could be found which departed from this perceptibly, and 

 none of these had a perfectly monoplanal disposition. The origin 

 of the tetrahedral arrangement can not be attributed to the homoe- 

 otyplc spindles, being from the first at right angles to each other. 

 Probably in somewhat less than 50 per cent, of the mother-cells 

 are they exactly at an angle of 90° during the anaphases. More 

 often they have an inclination of from 45° to 60°. However, 

 when the chromosomes have reached the poles, the four aggre- 

 gates which they compose are found to be almost invariably equi- 

 distant, tetrahedrally arranged, and appressed to the plasma mem- 

 brane (fig. 25). It is thus just before nuclear re-organization 

 that the tetrahedral arrangement is finally determined. Giesen- 

 hagen (24) has written extensively on the orientation of spindles 

 and division planes, attributing them largely to the polarity of the 

 mother-nucleus. 



As noted, the cell was at first perfectly spherical, but soon the sides 

 parallel with the spindles appear flattened (fig. 27), the whole cell 

 thus becoming a tetrahedron with each of its four faces lying over a 

 group of three nuclei and parallel to the spindles between them. 



