farr: cytokinesis of pollen-mother-cells 299 



projection of the mother-wall is triangular in section, the angles 

 which are, however, never sharp, representing the edges of the 

 furrows as they cut into the equator of the spindles. Figure 33 

 shows a section cut through the center of one of the nuclei of the 

 mother-cell and tangential to two others, showing the projection 

 of the mother-wall extending in to the center of the cell where it 

 has fused with those from the three other directions. 



By the fusion of the four projections of the mother-wall at the 

 center of the mother-cell the latter is divided into four uninucleate 

 protoplasmic masses, each connected with the other three by a 

 broad isthmus of cytoplasm through which spindle fibers continue 

 to pass. The furrows continue to deepen, as before, and the pro- 

 jection of the mother-wall enlarges transversely so that there is a 

 constriction of the isthmus between each pair of daughter cells. 



While these processes are going on, there is simultaneously a 

 migration of the nuclei away from the plasma membrane (figs. 

 28-37); until at the end of the division the nucleus occupies the 

 central position in the daughter cell. The conditioning factors 

 in such migration have, of course, not yet been determined with 

 certainty; it is, however, interesting to note that should the nuclei 

 be thought of as bearing electric charges of like sign, they would 

 behave as described: namely, assume a tetrahedral disposition 

 before cell-division. And if further we think of the plasma mem- 

 brane as bearing an electrical charge of sign unlike to that of the 

 nuclei, the nuclei would ultimately move to a central position in 

 the daughter cells, in other words to the position of equilibrium, 

 considering that they bear charges unlike that of the plasma 

 membrane. 



The tetrahedral arrangement of the nuclei, and consequently 

 of the microspores, seems to be the prevalent disposition among 

 dicotyledons and, in fact, among all of the higher plants, especially 

 those in which quadripartition has been found. Below is given a 

 table, showing certain data on this point given in the literature. 

 Of the forms studied in this investigation the predominant dis- 

 position in all is tetrahedral, though Chrysanthemum shows a 

 greater diversity in this respect than any of the other species 

 It will be seen that out of 50 forms of bryophytes, pterido- 

 phytes, and spermatophytes which have been studied in this 

 respect, 40 are reported to have the tetrahedral arrangement; in 



