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 Lilium, 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. 1 1 , 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 Lilium, 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 Trade- 

 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 ? 



