THE ACHROMATIC FIGURE, CYTOKINESIS, AND CELL WALL 177 



extreme cases wall formation may still be seen in progress al the periphery 

 after the fibers have completely disappeared al the central region (Fig. 

 58, E). Such is notably the case in the tangential divisions of elongated 

 cambium cells (Bailey 1919, 1920). 



The spindle in many cases has an origin somewhal differenl from thai 

 described above. The first indication of its differentiation is here the 

 appearance of a weft of fine fibrils in the cytoplasm all around the nucleus 

 (Fig. 84, E). As these fibrils increase in size and number they may 

 form several distinct groups extending in various directions, thus giving 

 a multipolar spindle (Fig. 58, F). Some of the groups then gradually 

 disappear, while others alter their positions and coalesce, so thai a bipolar 

 spindle eventually results. This, in general, is the manner in which the 

 spindle arises in the microsporocytes of angiosperms. For example, 

 Lawson (1898, 1900, 1903) finds that in Cobcea, Gladiolus, and Ins a 

 zone of granular "perikaryoplasm" collects about the nucleus during 

 the prophases of mitosis. When the nuclear membrane dissolves, this 

 substance together with the linin of the nucleus forms a fibrous network 

 which grows out into several cones of fibers, and these later become 

 arranged in two opposed groups. 



In Animals. — In the majority of animal cells, and in certain cells of 

 lower plants also, the achromatic figure is a much more elaborate struc- 

 ture than that of the higher plants described above. This is due to the 

 presence of centrosomes, which with their asters are very conspicuous at 

 the time of mitosis. Commonly the aster is not present during the resting 

 stages of the cell, but cases are known in which both centrosome and 

 aster are visible, forming with other materials an "attraction sphere'" in 

 the cytoplasm. As the process of mitosis begins (Fig. 59), an aster, if 

 not already present, develops about the centrosome. The centrosome 

 divides, and as the daughter centrosomes move apart each is seen to be 

 surrounded by its own aster, and a small group of fibers ("central spin- 

 dle") extends between them. The achromatic figure, made up of the 

 asters and the spindle connecting them, is known both at this stage and 

 later as the amphiaster. As the daughter centrosomes continue to sepa- 

 rate the astral rays increase in prominence. Some of the rays grow into 

 the nucleus when its membrane disappears and become attached as 

 mantle fibers to the chromosomes, while the lengthening central spindle 

 between the asters becomes the cent ral spindle port ion < connect ing fibei - 

 of the completed mitotic figure (Fig. 49). All the fibers focus upon the 

 centrosomes. 



During the anaphase the asters remain very conspicuous, but as the 

 telophases progress they gradually fade from view, except in those forms 

 which have a more or less permanent attraction sphere. Iside from the 

 presence of centrosomes and asters the achromatic figure in animal cells 

 differs most conspicuously from that of higher plant cells in its behavior 



12 



