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). 
