l8 HISTOLOGY 



which invade the nucleus, and also radiations which have been traced 

 even to the cell walls. But as Rosen states, sun-like figures, such as cer- 

 tain botanists have pictured, do not occur. Schaffner has described a 

 distinct centrosome or central granule in the root tip of the onion, but 

 Richards finds that in Podophyllum there is no such structure, and the 

 weight of evidence appears to be against the existence of a definite centro- 

 some in the higher plants. 



Anaphase. In the anaphase the halves of each chromosome move to 

 the opposite poles (Fig. 12, F). The figure thus produced is known as a 

 double star or diaster. Since each chromosome has divided into two, 

 the original number of chromosomes is preserved, and an equal number 

 of rods will be found in either star. They cannot all be brought into 

 focus together, and because of overlapping, they are hard to count. 

 Sometimes one chromosome, longer than the others, remains for a time 

 as a continuous bar from one aster to the other. Between the asters 

 there are always straight spindle fibers, but they vary in distinctness. 

 (The anaphase in an animal cell is well shown in Fig. 21, D.) 



Telophase. After the chromosomes have reached the opposite poles, 

 they form two dense masses. They are generally said to unite end to 

 end, thus forming a spireme thread. But in the root tips of Trillium, 

 Gregoire and Wygaerts state that they come into contact with one another 

 laterally; and as they separate, transverse connections are retained, 

 which, with the vacuolization of the chromosomes, restore the nuclear 

 reticulum. This may not be the correct interpretation, but immediately 

 after the anaphase the chromosomes form a very compact mass, easily 

 overstained so that it appears solid. Subsequently the mass enlarges 

 (Fig. 12, H), and the chromosomes become coarsely granular, taking the 

 form of wide bands. Nucleoli reappear, and according to Richards, 

 "it is a general rule that they arise on the side of the nucleus nearest the 

 new cell wall." This accords with Nemec's statement that they form 

 from the outer fibers of the spindle. Nemec and Rosen agree that they 

 first appear outside of the nucleus, which they enter before the nuclear 

 membrane develops. These are details which require confirmation. 



The new cell wall arises in plants as a series of thickenings of the 

 interzonal spindle fibers, which at this stage form a barrel-shaped bundle 

 (Fig. 12, G). The thickenings coalesce to form a membrane which does 

 not at first reach the sides of the cell. While this wall is developing the 

 nuclei are in a condition resembling the spireme stage of the prophase. 

 The entire mitotic figure is therefore called the double spireme or di- 

 spireme. The cell wall is soon completed and the nuclei return to the 

 resting condition (Fig. 12, I). 



The time required for mitotic cell division varies from half an hour 

 (in man) to five hours (in amphibia). After death, if the tissues are not 



