THE CELL 



55 



CHANGES IN A CELL UNDERGOING DIVISION 



The linin threads become thicker and shorter. The chro- 

 matin granules increase in size and amount; the threads and 

 chromatin granules separate into a definite number of segments 

 or chromosomes (Plate i, Fig. 2). The nuclear membrane be- 

 comes invested with a fibrous protoplasmic layer which later 

 separates and passes into either end of the cell, there forming 

 the polar caps (Plate 1, Fig. 3). 



The nuclear membrane and the nucleoli disappear at about 

 this time. Two fibres, one from each polar cap, become at- 

 tached to opposite sides of the individual chromosomes. Other 

 fibres from the two polar caps unite to form the spindle fibres, 

 which thus extend from pole to pole. All these spindle fibres 

 form the nuclear spindle (Plate 1, Fig. 5). 



The chromosomes now pass toward the division centre of 

 the cell or equatorial plane and form, collectively, the equatorial 

 plate (Plate 1, Fig. 5). At this point of cell division, the chromo- 

 somes are U-shaped, and the curved part of the chromosomes 

 faces the equatorial plane. The chromosomes finally split into 

 two equal parts (Plate 1, Fig. 6). The actual separation of the 

 halves of chromosomes is brought about by the attached polar 

 fibres, which contract toward the polar caps (Plate 1, Fig. 7). 

 The chromosomes are finally drawn to the polar caps (Plate 1, 

 Fig. 8). The chromosomes now form a rounded mass. They 

 then separate into Hnin threads and chromatin granules. Nuc- 

 leoli reappear, and nuclear sap forms. Finally, a nuclear mem- 

 brane develops. The spindle fibres, which still extend from 

 pole to pole, become thickened at the equatorial plane (Plate 1, 

 Fig. 8), and finally their edges become united to form the cell- 

 plate (Plate 1, Fig. 9), which extends across the cell, thus com- 

 pletely separating the mother cell into two daughter cells. After 

 the formation of the cell-plate, the spindle fibres disappear. 

 The cell becomes modified to form the middle lamella, on either 

 side of which the daughter protoplast adds a cellulose layer. 

 The ultimate composition of the middle lamella and the com- 

 position and structure of the cell wall will differ according to 

 the function which the cell will finally perform. 



