INDIRECT DIVISION 15 



Prophase. The first indication of approaching division is a change 

 in the chromatin, which becomes gathered into fewer and coarser granules 

 and takes a deeper stain. Portions of the linin network break down, so 

 that the chromatin granules come to be arranged in long convoluted 

 threads. Such threads are developing in the cell, Fig. 12, B, but are 

 more perfectly formed in C. It is possible that at a certain stage the 

 nucleus contains only a single continuous thread, but this condition can- 

 not be demonstrated in Tradescantia. The stage of nuclear division in 

 which the chromatic material appears to be arranged in a coiled thread 

 or skein is called a spireme. The "close spireme" (B) is succeeded by 

 the "loose spireme" (C). Successive stages in the development of the 

 spireme in animal cells are seen in Fig. 20, D, E, and F. 



As the spireme develops, the nuclear membrane becomes less distinct, 

 and the clear zones disappear from around the nucleoli. The nucleoli 

 become apparently less regular in outline, and forms which suggest that 

 two of them have fused (Fig. 12, B) are perhaps more frequently seen 

 than in resting cells. Usually it is stated that the nucleoli break up into 

 smaller bodies toward the time of their dissolution, and that some of 

 these escape into the cytoplasm after the disappearance of the nuclear 

 membrane. Farmer and Shove believe that the nucleoli contribute to 

 the chromatin; Richards regards them as a store of food material for the 

 rest of the cell; and others believe that they form the achromatic 

 "spindle" which will be described presently. Their function in animal 

 cells is equally uncertain. 



In the stage shown in Fig. 12, D, which may be regarded as the end 

 of the prophase, the nuclear membrane and the nucleoli have disappeared, 

 and the spireme thread has become divided into a number of segments 

 or chromosomes. These are straight or curved rods of different lengths. 

 Sometimes they appear as bent V-shaped bodies, but these often represent 

 two chromosomes with their ends together. J-shaped forms, with one long 

 and one short arm, have been described in various plants. The chromo- 

 somes become so arranged that one end of the rods, or the apices of the V's, 

 are situated in the equatorial plane, which extends transversely across the 

 middle of the cell. Often it is temporarily tilted (as in D and E) as if the 

 mitotic apparatus had shifted to a position in which it obtained more 

 space. It may do this mechanically if the contents of the cell are under 

 pressure. When the chromosomes are gathered at or in the equatorial 

 plane, they constitute collectively the equatorial plate. Because of their 

 stellate arrangement at this stage, which is best seen in transverse sections 

 of the cell, this mitotic figure is known as the aster. 



The manner in which the chromosomes are formed from the spireme 

 thread is difficult to determine. According to Gregoire and Wygaerts, 

 the linin and chromatin, which have often been regarded as closely related 



