32 INTRODUCTION. 



formation of the cell-plate. This method is, however, closely related 

 to cleavage. 



As an illustration of the method of cell-plate formation typical of 

 higher plants, the pollen mother-cells of Lilium furnish excellent 

 material. Here a cell-division follows the first nuclear division. The 

 connecting fibers are well developed, and with suitable fixing and 

 staining the details stand out with a clearness unequaled among plants. 

 As we have seen in Fig. 10, G, the daughter spirems are connected by 

 a beautiful system of connecting fibers, which is slightly barrel-shaped 

 at an early stage. The fibers soon show a thickening in the equatorial 

 region, which stains more intensely with gentian violet. The thicken- 

 ings are not granular or lumpy, but rather homogeneous, and are due 

 to the accumulation of kinoplasm, the substance out of which the 

 cell-plate, or plasma membrane, is made. At a little later stage 

 (Fig. 10, H) there appears in the central part of the system of con- 

 necting fibers in the region of the equator a fine homogeneous line, 

 the beginning of the cell-plate. This young cell-plate is evidently 

 in the form of a circular disk, which proceeds in growth uniformly 

 toward the periphery of the cell. The cell-plate is not necessarily 

 formed by the meeting or union of thickened places of the connecting 

 fibers, for in many cases the fibers are too far apart. The kinoplasmic 

 material is brought to the place occupied by the new plasma membrane 

 and there deposited in the form of a fluid substance. With the further 

 growth of the cell-plate the connecting fibers bulge out more and 

 more, being always thicker and more numerous at the outer edge or 

 surface of the system (Fig. 10, H). As the peripheral fibers of the 

 barrel-shaped system bulge out, its longitudinal axis becomes shorter, 

 so that the daughter spirems come eventually to lie in the center of 

 the daughter cells. In Fig. 10, I, the cell-plate is just complete, the 

 peripheral fibers which have reached the plasma membrane of the 

 cell being more numerous there. 



The cell-plate or plasma membrane is now seen to be double, and 

 it is the author's opinion that the new plasma membrane is formed 

 double. The fact that each daughter or granddaughter cell, when 

 somewhat shrunken at this stage, is seen to possess its own plasma 

 membrane, seems to support this view. 



Soon after the formation of the plasma membranes, a cell-wall is 

 deposited between them. Until the primordia of the daughter nuclei 

 (Fig. 10, J) are provided with a nuclear membrane, the chromatin 

 spirem is in the form of a circular disk from whose margin radiates a 

 zone of kinoplasmic fibers toward the equatorial edge of the cell. In 

 optical section this zone appears as a bundle of fibers on the right and 



