tend to line up more or less uniformly on the periphery of a circle or 

 ellipse bounding the central region of the spindle, with the chromosome 

 arms lying at random either within or without the spindle area (Figure 

 5-1). Each chromosome is independent of the others and in most cases 

 its position bears no discernible relation to that of its homologue. 



Anaphase 



This is the stage of separation of half chromosomes, or chromatids, 

 which move toward opposite poles, generally with the kinetochore first 

 and the arms trailing. 



Telophase 



On arrival at the poles the chromosomes progressively lose their 

 chromaticity and indications of internal structure can often be seen, 

 though individual chromosomes can rarely be resolved as separate struc- 

 tures. Reformation of the membrane, nucleolus, and chromocenters 

 brings us back to the interphase stage again, with the result that one 

 nucleus has given rise to two which are its genetical and cytological 

 equivalents. 



In most cases in plant cells, a cell wall is formed in the region of the 

 equatorial plate to form two daughter cells. In animal cells, daughter 

 cells are usually separated by a process of invagination in the equatorial 

 region. Electron microscope studies of Porter and Machado (1960) 

 indicate that formation of the cell plate is initiated by migration of tubu- 

 lar elements of the endoplasmic reticulum toward the interzonal region 

 of the spindle where thev spread out to form a close network along the 

 equator of this region. This network marks the midline of the develop- 

 ing phragmoblast within which the cell plate forms. The cell plate 

 appears first as a series of unconnected vesicles within the meshes of the 

 network, which later increase in size and fuse to complete the separa- 

 tion of daughter cells except at points where continuity is maintained 

 by the plasmodesmata (Figure 5-5). The present concepts concerning 

 cell wall formation in plant cells have recently been reviewed by 

 Whaley and coworkers (1960). 



It must be remembered that the process described here is a dynamic 

 affair and that one stage merges into the next without any definite line 

 of demarcation. The names attached are useful only in that they provide 

 a means of designating the part of the process under discussion. 



It may also be noted that this description is a generalized one and 

 that there are many variations, some of which are normal in that they 

 occur characteristically in certain tissues of special organisms. Any part 



124 / CHAPTER 5 



