8 Mitosis in Pellia 



of an astral ray. The structure at the upper pole in Fig. 9 certainly looks like a cen- 

 trosphere containing a centrosome, but such an appearance is so rare that it seems 

 safer to regard the sharply staining body as a chance granule. Still, it is evidently 

 just such a body as this that Van Hook (38), in his recent study of Marchantia, 

 interprets as a centrosome. 



In the very early prophases a beautiful system of radiations becomes quite con- 

 spicuous. This system we regard as an aster, comparable with the asters of Thal- 

 lophytes and of animals. The system first appears as a few fibers converging to a 

 point which is usually in contact with the nuclear membrane or very near to it (Figs. 

 11-13), but, in some instances, may be at a considerable distance from the nucleus 

 (Figs. 14-16). Persistent search failed to reveal any body which could be identified 

 positively as a centrosome or centrosphere before the appearance of the aster, and 

 even after the appearance of the aster and centrosphere, no centrosome could be dis- 

 tinguished. Granules, like those shown in all the figures, were frequently found in 

 contact with the nuclear membrane after the nucleus had begun to enlarge, and it is 

 probable that some of the granules were centrospheres, although no method was found 

 for identifying them before the appearance of the rays. Bodies which bear remark- 

 able resemblance to centrosomes (Figs. 14, 16, 17) and which, for a time, were inter- 

 preted as genuine centrosomes, proved to be merely the cut ends of coarse fibers. 

 Sometimes several deeply staining points may be seen; such an appearance might 

 easily be mistaken for a centrosphere containing several granules. In cases like those 

 shown in Figs. 14-17, the "granules" are, without doubt, nothing but the cut ends of 

 fibers. The two centrospheres in Fig. 17 are practically alike, but the one at the 

 upper pole is represented in median section and the other in surface view, the fibers in 

 vertical view appearing as dots. However, it must be admitted, that occasionally the 

 deeply staining points are really granules (Fig. 9), but the cases are so rare that we 

 have not regarded such granules as a functional part of the mitotic mechanism. 



After a study of the germinating spore had failed to show any centrosomes, the 

 nuclear figures were examined in other phases of the life-history, particularly in the 

 apical cell and its younger segments, and in the developing antheridia. The apical 

 cell and the rapidly dividing cells near it are quite favorable for study. The charac- 

 ter of the mitoses in this region is represented in Figs. 9 and 10. The lower pole in 

 Fig. 9 shows the more usual condition, although the rays are frequently as strongly 

 developed as those shown at the upper pole, a considerable number of the rays reach- 

 ing to the Hautschicht. A careful examination of this figure will show that there is 

 no definite centrosphere like those in Figs. 12 and 13. In later stages (Fig. 10) the 

 spindle becomes sharply bipolar and the radiations disappear. 



The antheridia were examined with particular interest because Schottlander (30) 

 had reported centrosomes during all stages in the development of the antheridium of 

 Marchantia, and Belajeff (2) had found blepharoplasts throughout the development of 

 the spermatogenous cells of Marsilea. However, nothing which could be interpreted 



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