MITOSIS IN POLLEN MOTHER-CELLS. 15 



We shall now leave the chromosomes for the present and pass to 

 the development of the spindle. 



DEVELOPMENT OF THE SPINDLE. 



The development of the spindle in pollen mother-cells varies some- 

 what in detail in different plants, but it can usually be referred to one 

 type. In all cases, so far as known, it arises as a multipolar structure. 

 As soon as the spirem is segmented into chromosomes, and some- 

 times earlier, the kinoplasmic fibers make their appearance in the cyto- 

 plasm. The arrangement of the kinoplasmic fibers is not quite the 

 same in all cells of the same anther. They may be disposed at first 

 radially about the nucleus (Fig. 6, A), or, as in many cases, may form 

 a weft about the nucleus midway between nuclear membrane and cell- 

 wall (Fig. 6, B). The remaining cytoplasm consists of a fibrillar 

 structure. In this stage the nucleus is filled with a fluid which does 

 not stain, namely, the nuclear sap. The chromosomes are connected 

 with each other and with the nuclear membrane by means of fine 

 fibers, and one or more nucleoli are present. The nucleolus, how- 

 ever, begins to break up at this time, so that one large and several 

 smaller ones may be present. 



The next step in the development of the spindle may differ slightly 

 in different cells, owing to the orientation of the kinoplasmic fibers. 

 In those cells in which these fibers are disposed radially about the 

 nucleus, the tendency to form poles manifests itself before the disap- 

 pearance of the nuclear membrane. Groups of radiations converge 

 toward various points near the plasma membrane, while others form a 

 weft about the nucleus (Fig. 6, C). A little later the nuclear mem- 

 brane is replaced by this weft, and the fibers begin to enter the nuclear 

 cavity. In some cases well-defined poles (or only a few) are not as 

 yet present. In other cases a greater number of poles are formed, and 

 we have then a very remarkable multipolar complex of kinoplasmic 

 fibers surrounding the nucleus, into which the fibers penetrate from 

 all sides (Fig. 7, E). 



Gradually more kinoplasmic fibers enter the nuclear cavity until it 

 can no longer be recognized as such (Fig. 6, D). In this complex of 

 spindle fibers the chromosomes are irregularly distributed. They are, 

 however, soon collected together, and to each a bundle of fibers be- 

 comes attached. The chromosomes seem to be aggregated more closely 

 together by a pushing and pulling of the spindle fibers. Owing to the 

 irregular arrangement of the chromosomes and the complexity of the 

 mass of spindle fibers, it is not always possible to determine at this 

 stage the exact manner in which the fibers are fastened to the chro- 

 mosomes (Fig. 7, F). 



