6 8 CELI.-DI I ISION 



bodies in various eggs) the nucleolus is cast out into the cytoplasm as 

 the spindle forms, to persist as a " metanucleus " for some time before 

 its final disappearance (Fig. 104). More commonly the nucleolus 

 fades away /// situ, sometimes breaking into fragments meanwhile, 

 while the chromosomes and spindle are forming. The fate of the 

 material is in this case only conjectural. An interesting view is that 

 of Strasburger ('95, '97), who suggests that the true nucleoli are to be 

 regarded as storehouses of " kinoplasmic " material, which is either 

 directly used in the formation of the spindle, or, upon being cast out 

 of the nucleus, adds to the cytoplasmic store of " kinoplasm " avail- 

 able for future mitosis. 



(/;) The Auiphiastcr. Meanwhile, more or less nearly parallel with 

 these changes in the chromatin, a complicated structure known as the 

 aiuphiastcr {¥o\, '77) makes its appearance in the position formerly 

 occupied by the nucleus (Fig. 25, B-F). This structure consists of 

 a fibrous spindle-shaped body, the spindle, at either pole of which is 

 a star or aster formed of rays or astral fibres radiating into the sur- 

 rounding cytoplasm, the w^hole strongly suggesting the arrangement 

 of iron filings in the field of a horseshoe magnet. The centre of each 

 aster is occupied by a minute body, known as the centrosome (Boveri, 

 '88), which may be surrounded by a spherical mass known as the 

 centrosphere {^\x2J^\\xgQ.\, '93). As the amphiaster forms, the chro- 

 mosomes group themselves in a plane passing through the equator of 

 the spindle, and thus form what is known as the equatorial plate. 



The amphiaster arises under the influence of the centrosome of the 

 resting cell, which divides into two similar halves, an aster being 

 developed around each while a spindle stretches between them (Figs. 

 25, 27). In most cases this process begins outside the nucleus, but 

 the subsequent phenomena vary considerably in different forms. In 

 some forms (tissue-cells of the salamander) the amphiaster at first lies 

 tangentially outside the nucleus, and as the nuclear membrane fades 

 away, some of the astral rays grow into the nucleus from the side, 

 become attached to the chromosomes, and finally pull them into posi- 

 tion around the equator of the spindle, which is here called the cen- 

 tral spindle (Figs. 25, D, F; 27). In other cases the original spindle 

 disappears, and the two asters pass to opposite poles of the nucleus 

 (some plant mitoses and in many animal-cells). A spindle is now 

 formed from rays that grow into the nucleus from each aster, the 

 nuclear membrane fading away at the poles, though in some cases it 

 may be pushed in by the spindle-fibres for some distance before its 

 disappearance (Figs. 25, 32). In this case there is apparently no 

 central spindle. In a few exceptional cases, finally, the amphiaster 

 may arise inside the nucleus (p. 304). 



The entire structure, resulting from the foregoing changes, is 



