MITOSIS IN POLLEN MOTHER-CELLS. 1C) 



the points of the crescents .to form ring-like chromosomes (Fig. 9, D, 

 at the right). In the majority of cases, however, they adhere at only 

 one end, and under such circumstances each chromosome consists of 

 two thick and slightly curved pieces placed end to end, and as they 

 are oriented tangentially upon the spindle, reach nearly from pole to 

 pole (Fig. 9, D). 



The chromosomes in Podophyllum present the same variety of forms 

 found in Lilium and Tradescantia. Here the segments may be in 

 close contact, side by side, or form loops, rings, X's, and Y's. Per- 

 haps the majority of chromosomes in Podophylhim present the form 

 last mentioned for Tradescantia. 



In Lilium the chromosomes, when in the nuclear plate, are usually 

 arranged with much regularity about the periphery of the spindle. 

 The majority are fastened to the fibers at the ends, and stand radially 

 to the axis of the spindle (Fig. 7, H). When observed from the pole 

 in this stage, they are seen to radiate like the spokes of a wheel from 

 the central spindle fibers. But all the chromosomes are not so regu- 

 larly oriented upon the spindle, and their manner of attachment to the 

 fibers is also variable. As will be seen in Fig. 8, F-K, they may be 

 fastened to the spindle at some distance from one end or near the mid- 

 dle. Those that are quite regularly ring-shaped are attached near the 

 middle of each segment. In all these cases, the chromosomes are 

 placed tangentially upon the spindle. The X-, Y-, and loop-shaped 

 chromosomes are usually fastened to the spindle as indicated in Fig. 

 8, F, J, K. Karyokinetic figures are not rare in which two or more 

 of the different forms of chromosomes, with their different orientations 

 and different methods of attachment to the fibers, are found in the 

 same spindle. 1 



The stage of the mature spindle persists some time and evidently 



1 Other interpretations of the chromosomes appearing in the first mitosis have been given by different 

 observers and by the same investigator at different times, owing to the trend of theoretical considerations. 

 One of these, which was announced as early as 1884 by Heuser for Tradescantia virginica (Beobach- 

 tung uber Zellkerntheilung. Bot. Centralblt., 17 : 1884) and which has very recently received support by 

 Strasburger and others (Ueber Reduktionstheilung. Sitzbr. der Konig. Preuss. Akad. der Wiss., 18 : 

 1-28, 1904) is that the two segments of each chromosome appearing in the equatorial plate of the first 

 mitosis are not the result of the longitudinal splitting of the spirem occurring in the early prophase, but 

 are formed by the folding together or approximation of two chromosomes, each consisting of the two 

 daughter segments resulting from the longitudinal splitting. Each chromosome is therefore a bivalent 

 chromosome, and the first or heterotypic mitosis is a qualitative or reducing division, whereas the second 

 mitosis is equational, the segments separating along the line of the longitudinal split. Strasburger bases 

 his conclusion mainly upon data obtained from studies of the pollen mother-cells of Galtonia candicans. 

 The figures which he gives in support of this view in the paper cited seem to me to be far from convinc- 

 ing. Moreover, Jules Berghs, in a recent study of the prophase of the heterotypic mitosis in Allittm 

 fistulosum and Lilitimlancifoliutn (speciosum) (La Cellule, 21: 173-188, 1904), shows clearly, in a careful 

 series of stages, that the two segments of each chromosome are the result of the longitudinal fission and 

 not that of a folding together or approximation of two chromosomes. Unfortunately the papers cited 

 reach me too late for further consideration, as these pages are already in press. 



