10 Mitosis in Pellia 



third mitosis we never found anything but the solitary aster in the earliest stages. In 

 studying this point, reconstructions were made from thin sections, and series were cut 

 thick enoufifh to include the entire nucleus. It is true that the first aster does not 

 usually reach its fullest development before one appears at the opposite pole. In Figs. 

 14 and 15 and also in Fig. 19 (third division) there is only one aster. However, the 

 second aster usually appears before development has proceeded so far. In spite of the 

 fact that the two asters do not arise simultaneously, we can confidently support Davis's 

 (7) conclusion that the two asters do not arise by the division of a single one. We found 

 only two preparations in which the asters were less than 180° apart, except in case of 

 tripolar figures, which were not very rare (Fig. 16, Plate XXVI — third pole not shown — 

 and Plate XXVII, Fig. 23). In early stages the two poles usually differ from each other 

 in appearance, one pole being rather pointed and the other comparatively blunt (Plate 

 XXVII, Figs. 21, 22, 24, 25). Cases like Fig. 21 indicate that the blunt pole has been 

 the last to develop. At this stage, neither pole is sharp, both being more or less 

 rounded. The dome-shaped prominences or " caps," as they may be called, are by no 

 means easy to interpret. In some cases the cap looks like a mere extrusion of the 

 nuclear membrane, while in others the nuclear membrane is still intact after the caps 

 have become quite conspicuous. The rounded ends indicate considerable pressure from 

 beneath. That the cap is something more than a structure built up by fibers radiating 

 from the aster is shown by its appearance and by the fact that in transverse section 

 it presents a continuous line. The cap becomes finely granular and suggests a delicate 

 membrane being resolved into fibers, rather than a membrane being formed from fibers 

 (Fig. 4). In our opinion, the cap is derived from the outer portion of the nuclear 

 membrane, or is itself a delicate layer — a sort of Hautschicht — immediately sur- 

 rounding the nuclear membrane. The caps do not seem to be different from those 

 see-n in the root tips, as described by Nemec (24), Schatfner (28), and others. 



The rays of the aster do not penetrate the caps, but are closely applied to them. 

 The aster exerts a strong pull, as may be seen during the period of elongation, although 

 the elongation is due, in some degree, to pressure from within. 



As in the first mitosis, the spindle in early stages consists of two half-spindles 

 (Fig. 26). Until the caps become resolved into fibers they keep the spindle 

 rounded (Fig. 26). The caps generally break up into fibers during the metaphase or 

 early anaphases, and the poles of the spindle may then become blunt or irregular (Fig. 

 27). Occasionally the caps keep the poles of the spindle rounded even after rather 

 late anaphases have been reached (Fig. 28). 



The polar radiations generally disappear at the end of the prophases, are absent 

 during the metaphase and anaphases, and reappear in the telophase (Figs. 26-29). 

 That portion of the spindle which lies between the two caps is undoubtedly nuclear in 

 origin. It consists of a very dense mass of spindle fibers which appear with remarka- 

 ble suddenness (Fig. 20; cf. Fig. 5). 



From observations on the nucleolus, we feel sure that this body contributes consid- 



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