MITOSIS 



FlG. 27. Hyacinth root-tip cell with spireme 

 formed. Nucleolus dissolving. 



Next to take place is a shortening of the spireme and a thickening 

 of its diameter, accompanied by a merging or blending of the constituent 

 chromatin granules into each other 

 so that the spireme assumes a 

 smooth, wire-like appearance (Fig. 

 27). Other phenomena soon take 

 place. Two clusters of transpar- 

 ent filaments appear in the cyto- 

 plasm, one on either side of the 

 nucleus and opposite each other. 

 Each cluster consists of many 

 fibrils of an achromatic substance 

 (Fig. 28). These fibrils are scat- 

 tered evenly and closely on the 

 nuclear membrane, about one 

 fourth of whose surface they 

 cover. They reach from there 

 outward toward a common center in the cytoplasm, at which they 

 converge. Thus each cluster of fibers forms a short, thick cone, 

 placed opposite to its neighbor, and the two together form an in- 

 complete diamond- shaped outline, 

 which is called the spindle when a 

 little further developed. Shortly 

 after the spindle has begun to 

 appear, the nuclear membrane 

 begins to dissolve, becoming thin- 

 ner, and finally disappearing at 

 one or more points, but without 

 losing at any time its rigid curved 

 outline. It apparently dissolves in 

 situ, and when once started, the 

 breaches rapidly widen until all 

 the membrane is gone. Its points 

 of first disappearance are usually, 



ST^X^J^-'V^^^'^T' N Vff k ut not necessarily, at the poles 

 x^Mfei;li&^lr^/ as indicated by the spindle. 



Shortly after the membrane 

 has gone at these poles the spindle 

 fibers grow into the nucleus and 

 touch the chromatin spireme. The 

 nucleus then develops into the 



somewhat more frequently seen stage pictured in Figure 29. Here 

 three changes are apparent. The nuclear membrane is completely 



FIG. 28. Hyacinth root -tip cell with spindle 

 forming and nuclear membrane dissolved at 

 two points. Nucleolus appears larger, but 

 is smaller in bulk, and apparent size is due 

 to vacuoles. 



