1/2 GRACE MEDES. 



The formation and appearance of the spindle fibers up to the 

 time of the metaphase has been described above (Fig. 16). Dur- 

 ing this period they lie like heavy black threads in the transparent 

 substance derived from the nucleus (Fig. 17), one fiber being 

 attached to each chromosome and so distinct that it may be traced 

 from one end of the spindle to the other. The net work through- 

 out the cell, except in the immediate vicinity of the centrosomes, 

 is of the same regular nature characteristic of the cell during 

 the resting stage, and shows the same density marking it through- 

 out the prophase. Around the centrosomes, however, the astral 

 rays are beginning to elongate and strengthen. At the same 

 time the net-work surrounding the spindle becomes more ragged, 

 the cytoplasm through the equatorial plane lighter and freer 

 from linin until by the time the chromosomes have reached their 

 destination at the extremities of the spindle fibers (Fig. 19) the 

 entire area in the vicinity of the spindle is clearer, and scarcely 

 any trace of the net-work can be discerned. Gradually now the 

 spindle fibers spread across the equatorial plane (Fig. 20). As 

 those upon the outside approach the periphery of the cell, where 

 the constriction of the wall has already commenced, they become 

 shorter and weaker as their extremities become dissolved in the 

 cytoplasm through which they are scattered. At no time are 

 the astral rays strong nor do they ever consist of heavy distinct 

 fibers like those of the spindle, but they appear much lighter 

 and at the extremities become lost in the net-work of the cyto- 

 plasm. They extend between the centrosomes and the equa- 

 torial plate but they never cross in this region. Indeed, scarcely 

 ever can they be distinguished this far, but fade into the network 

 of the cytoplasm about half way from the centrosomes to the 

 equatorial plane (Fig. 17). However, the whole cytoplasm of the 

 cell, extending from the centrosomes at the poles to the con- 

 stricting cell-wall at the sides, soon becomes transparent and 

 entirely free from net-work, apparently indicating some influence 

 of the centrosomes on this area (Fig. 20). 



As the constriction of the cell-wall continues, the regular net- 

 work of the cytoplasm begins to re-form about the periphery, 

 and the spindle fibers lying in the equatorial region become 

 pushed together until they form a sheaf-shaped bundle, the ends 



