166 CELL DIVISION : DEVELOPMENT OF GERM-CELLS 



The chromosomes now arrange themselves about a plane 

 (equatorial plane) at right angles to the axis of the spindle, 

 thus~forming the equatorial plate. 



The entire figure thus formed by the achromatic spindle 

 and the chromosomes is known as a mitotic figure. 



2. Metaphase. 



The preceding changes are preparatory to division, which 

 now begins by a longitudinal splitting of each chromosome into 

 two equal halves. In some cases the chromosomes split only 

 after they have taken up their position at the equator of the 

 spindle. This stage of division is known as the metaphase. 

 In other cases the division of the chromosomes occurs before 

 the equatorial grouping of these bodies. In either case the 

 original chromatin becomes divided into exactly equivalent 

 halves. 



3. Anaphase. 



After the chromosomes have split, one half of each chromo- 

 some travels along the spindle-fibre to one pole of the spindle, 

 the other half travels to the opposite pole. The result is 

 a group of daughter chromosomes at each pole of the spindle. 



4. Telophase. 



The entire cell now divides into two by a plane passing 

 through the equator of the spindle, each of the daughter cells 

 receiving an equal number of similar chromosomes, half of the 

 spindle and one of the asters with its centrosome. 



The chromosomes now lose their individuality and reconstruct 

 a fresh nucleus. The spindle disappears together, in most 

 cases, with the centrosomes and asters. In a few cases, 

 however, the latter persist. 



This process often takes place rapidly (in about one or two 

 hours), and usually during the night or early morning hours. 

 Between one division and the next there is a long pause. 

 Hence in any section the majority of cells do not show the 

 process of division unless the tissue has been prepared during 

 the critical period. - 



