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, /. ., 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 Lilium, 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 Lilium, PodopkyUum 

 and in many other higher plants : 



