MITOSIS IN POLLEN MOTHER-CELLS. 17 
quite regularly arranged in the equatorial plate. They are usually 
radially disposed, standing at right angles to the axis of the spindle’ 
(Fig. 7, H). The spindle fibers present the following arrangement : 
to each chromosome are attached two bundles of fibers (one to each 
daughter segment) which extend to the poles; other fibers, the central 
spindle fibers, run uninterruptedly from pole to pole, and still others 
diverge from the poles toward the cell periphery. This arrangement 
is commonly found in all cells of the higher plants, whether they be 
reproductive or vegetative. The spindle does not, as may appear at the 
first glance, present a system of meridional fibers converging toward 
the poles, but, as is easily seen from thin sections, the fibers cross and 
anastomose, giving the impression that the spindle consists of a weft or 
complex of fibers drawn out in the direction of the poles, which, indeed, 
it really is. 
In spore mother-cells of plants, the spindle fibers seem to be gener- 
ally of cytoplasmic origin, ¢. e., they appear first in the cytoplasm, 
forming a weft about the nucleus or radiating from it. In the 
generative cell of gymnosperms and in the first division following 
fecundation in these plants, it seems that the fibers or many of them 
arise from kinoplasm, which is in the nucleus or which entered the 
same in another form. 
CHROMOSOMES. 
As is well known, the chromatin spirem, which has split longitudi- 
nally in the early prophase, segments by transverse division into twelve 
chromosomes, the reduced number, or half the number in the vegeta- 
tive cells of the sporophyte. Each chromosome consists, therefore, 
of two daughter segments, or daughter chromosomes, which are 
almost always twisted upon each other (Fig. 7, H; Fig. 8). After 
the segmentation of the spirem into chromosomes, these contract, 
thereby becoming shorter and thicker. Previous to the disappear- 
ance of the nuclear membrane, they lie near it or are scattered 
throughout the nuclear cavity (Fig. 6, B). In Zedéwm, the daughter 
chromosomes are, as a rule, closely applied to each other, but in 
many Cases they tend to become separated soon after segmentation, so 
that various forms of chromosomes result, such as rings, loops, X- and 
v-shaped forms, depending upon the manner in which the daughter 
segments are oriented toward each other (Fig. 8, A to K). These 
various forms persist and may be found in the nuclear plate of the 
mature spindle. 
The following will explain the manner in which the more fre- 
quently occurring forms are brought about in Lzdéum, Podophyllum 
and in many other higher plants: 
