6 Mitosis in Pellia 



series at the first division. In fact, we have used Figs. 7, 8, and 19 of the third mito- 

 sis to illustrate also the same stages in the first and second mitoses. Nevertheless, it 

 is true that, in a great majority of cases, kinoplasmic activity is most energetic during 

 the first division, and that in succeeding divisions it becomes less and less conspicu- 

 ous until centrospheres and asters cease to attract any attention, and it finally becomes 

 doubtful whether they are present. 



THE FIRST MITOSIS IN THE GERMINATING SPORE 



As the nucleus of the germinating spore increases in size preparatory to the first 

 division, the area immediately surrounding it becomes comparatively free from starch 

 grains and coarser granules (Plate XXV, Fig. 1). It seems reasonable to suggest that 

 some substance, escaping from the nucleus into the cytoplasm, causes this zone and acts 

 as a stimulus to the formation of the extra-nuclear portions of the achromatic figure. It 

 is not impossible that such a substance might actually take the form of a centrosphere. 

 (The origin of the aster will be considered when dealing with the second division.) 

 After the spirem has become segmented into chromosomes the nucleus elongates and 

 the nucleolus appears very much vacuolated (Fig. 2). At this stage a pair of dome- 

 shaped caps (Figs. 3, 4) may be recognized at opposite poles of the nucleus. These 

 caps, ^hich will be considered later, appear in transverse section as a delicate ring, but 

 a similar section of the completed spindle shows a dense mass of fibers (Fig. 5). 



During the earlier prophases the poles of the spindle are usually rounded (Figs. 

 "B, 4, 6), but, as the metaphase approaches, the caps (Figs. 3, 4) which have given the 

 poles of the spindle a rounded form become resolved into fibers, and the poles may vary 

 in shape from sharply pointed figures, like that shown in Fig. 10, to such broad, indefi- 

 nite ones as those shown in Fig. 8 (Plate XXV) and Fig. 27 (Plate XXVII). Spindles 

 with three and even more poles are not very rare. They do not originate like the 

 multipolar spindles of the spore mother-cells of vascular plants, but are preceded 

 by the bipolar condition or are formed through the influence of three or more centro- 

 spheres or asters (Plate XXVI, Fig. 16, Plate XXVII, Fig. 23). During the anaphases 

 the poles of the spindle are sometimes sharp and sometimes indefinite. 



In the prophases it is plain that the achromatic figure is made up of the asters 

 and two half-spindles (Fig. 6). As the spindle continues to develop, some of the fibers 

 — the mantle fibers — become attached to the chromosomes; the other fibers increase 

 in length until they reach the opposite pole, thus forming a part of the central spindle. 



While the poles are separating from each other, radiations are easily seen, and 

 they continue to be fairly conspicuous until the spindle has reached its full length, 

 when they rapidly disappear, losing their staining capacity first at the peripheral ends, 

 then throughout their entire length, and finally becoming indistinguishable. When 

 the metaphase is reached, the radiations have usually disappeared (Fig. 7), and during 

 the anaphases, while the chromosomes are passing to the poles, it is very seldom that 

 any trace of radiations can be found. In the telophases, however, the radiations 



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