Division of Plant Cells 7 



these five steps are frequently recognized: propliase, prometa- 

 phase, metaphase, anaphase, and telophase. 



Prophase. During the resting or metabolic stage, the chromo- 

 somes are so long and thin and so intertwined that they cannot 

 be counted, and there is evidence that each is a single thread 

 until the cell is about to begin to divide (Fig. 2). With the 

 beginning of mitosis, however, a series of profound changes in 

 the nature of the nucleus is begun. There are a shortening and 

 thickening of the chromosomes and a probable loss of water and 

 increase in staining capacity of the individual threads, and if 

 the chromosomes in the resting nucleus are connected by branches 

 or anastomoses, as is frequently believed, these anastomoses are 

 withdrawn at this time. As the result of these changes the 

 individual chromosomes are more readily seen than in the rest- 

 ing nucleus and are no longer joined together in a reticulum. 

 One marked feature of the chromosomes in early prophase is 

 that they are double rather than single threads. They appear 

 as two long threads lying parallel and close to one another, each 

 of which contains a specialized region known as the centromere^ 

 kinetochore, or spindle fiber attachment point. In early pro- 

 phase, the chromosomes are still long and slender and still wind 

 about in a number of loose coils. 



As prophase progresses, the chromonemata uncoil and become 

 thicker. The matrix begins to condense on the threads, and the 

 chromosomes at this stage frequently have a fuzzy outline as the 

 result of the irregular accumulation of this deeply staining 

 matrical material along the length of the chromosome. 



The two threads that constitute the prophase chromosome are 

 the chromatids, each consisting of a chromonema and matrix. 

 The two chromatids are generally visibly uniform throughout 

 except for the centromeres, and the parts on either side of the 

 centromeres are called the arms. The region of the arm nearest 

 the centromere is the proximal region; the part farthest away is 

 the distal region. As prophase progresses, the matrix continues 

 to collect around the chromatids until each chromatid is now 

 a long, rod-like body lying next to its sister chromatid and 

 apparently identical with it in every way. The two centro- 

 meres lie side by side and in close contact. As these changes 

 occur in the chromosomes, the nucleolus or nucleoli get smaller 

 and smaller and at about the end of prophase have usually com- 



