Division of Plant Cells 9 



pletely disappeared. Towards the end of prophase, the chromo- 

 somes have become much shorter and thicker and stain much 

 more deeply than in the earlier stages. They also tend to move 

 towards the outer part of the nucleus. At this time the nuclear 

 membrane dissolves, and with the disappearance of this boundary 

 between the nuclear sap and the cytoplasm, prophase comes to 

 an end. 



Prometaphase. When the nuclear membrane disappears, the 

 nuclear sap and cytoplasm are brought into direct contact, and 

 the cytoplasm appears to act upon the nuclear sap so as to 

 cause it to form into a long, spindle-shaped structure known 

 as the spindle. In living cells, this structure is not easy to see, 

 but in many fixed and stained cells it appears as a number of 

 fine lines converging to two points. Earlier cytologists believed 

 these lines to be fibers and regarded the spindle as composed 

 of a number of such fibers, which were fairly widely separated 

 in the center of the spindle but converged at the ends. This may 

 be the correct interpretation, but the microdissection studies of 

 Chambers have tended to show that these so-called fibers are 

 not solid. 



Whatever is the correct nature of the spindle, it is a firmer, 

 more rigid structure than the cytoplasm in which it is embedded. 

 If the living cells are detached from one another and mounted 

 on a slide, the spindle is crushed only with difficulty, and the 

 cells generally lie so that the spindle is parallel rather than 

 perpendicular to the surface of the slide. The spindle is of great 

 importance in cell division and, if it fails to function properly, 

 mitosis will be abnormal. 



The spindle tapers at each end and may or may not come to a 

 sharp point. The ends are called the poles, and the region 

 equidistant between them, the equator. When the spindle 

 has formed, the chromosomes released by the breakdown of the 

 nuclear membrane move towards the equator. 



Metaphase. At metaphase, the chromosomes are seen to lie 

 on the equator of the spindle. They frequently arrange them- 

 selves so that they lie on the outer part of the spindle with only 

 the centromeres on the equator but sometimes, especially when 

 they are small and numerous, the chromosomes are found in 

 the center as well as in the outer region of the spindle (Fig. 3). 

 The centromeres always lie on the equator, forming an equatorial 



