MITOSIS IN POLLEN MOTHER-CELLS. 29 
ment, the two segments are separated from each other, being in con- 
tact only at the ends which are attached to the spindle fibers. Under 
this circumstance one segment may lie tangentially on one side of the 
equator and the other on the other. Other instances are observed also 
in which the two segments may lie parallel in pairs, but not in contact 
when arranged in the nuclear plate or at an earlier stage. Such cases 
as the two last mentioned would seem to indicate that the spirem, or a 
part of it, is segmented into the granddaughter chromosomes, and that 
these are then brought together in pairs. It is also probable that pieces 
of the segmented spirem, which are nearly straight, or only a little 
curved, may consist of two granddaughter segments, and these are 
brought side by side by the folding of the piece at or near the middle, 
so that the free ends are brought into apposition, after which the 
piece is severed at the point of bending. From a careful study of the 
second mitosis in the pollen mother-cells of Liléwm, Podophyllum, 
Tradescantia and others, the author is inclined to believe that the 
spirem may segment in the different ways just mentioned. However, 
the daughter spirem segments transversely into the granddaughter 
chromosomes, and during the development of the spindle these are 
arranged more or less in pairs in the nuclear plate (Fig. 11, C). 
In the nuclear plate, the chromosomes are oriented either radially, 
obliquely, or tangentially to the major axis of the spindle. The 
segments may be straight or variously bent, and, in either case, fre- 
quently twisted upon each other. In L7téwm, the segments are 
frequently, perhaps in the majority of cases, variously twisted, kinked 
or knotted, so that they can be followed for only a part of their 
length. In many cases, the kinked and twisted chromosomes seem 
to be so contracted as to form lumps. This is true also in 7rade- 
scantia and in numerous other plants. The bent, kinked, and 
twisted condition of the chromosomes seems to be due to the irregu- 
larity of the spirem, for it seems probable that, were all the turns of 
the chromatin skein regular and uniform, the greatly entangled nature 
of the spirem would not appear during the development of the spindle. 
We have seen that the identity of the individual chromosomes is 
lost from observation in the daughter spirem, and the question bear- 
ing upon the theory of the individuality of the chromosomes, naturally 
arises as to whether the chromosomes of the second, or homotypic 
mitosis, are-identical with the pairs of granddaughter segments of the 
anaphase of the preceding, or heterotypic division. In other words, 
are the two segments of each chromosome, appearing in the nuclear 
plate of the second nuclear division, sisters? Or may it be possible 
that some are sisters, while others are composed of segments from 
different pairs of granddaughter chromosomes of the first division? 
