24 ZOOLOGY 



The more striking stages in the process as it usually occurs 

 are outlined in the text and figures which follow. The centro- 

 somes and nucleus will be seen to be especially active. Such 

 a description is only typical of the remarkable series of events. 

 There are numerous variations from this in different organisms. 



1. In the quiescent or resting stage the structural elements, 

 are distributed in the way which is characteristic of the par- 

 ticular cell under examination (Fig. 8, A). The majority of 

 cells are found to be in this stage when examined. 



2. When division is about to take place, the chromatin 

 elements in the network of the nucleus assume the appearance 

 of a coil or tangle of thread (Fig. 8, B). The nuclear membrane 

 often breaks up at this time. 



3. The centrosome divides and the halves migrate toward 

 opposite poles of the nucleus, and from them as centres radia- 

 tions pass into the cell body in all directions. Across the 

 nucleus, from one centrosphere to the other, thread-like lines 

 ultimately extend, producing the appearance of a spindle (Fig. 

 8, C, sp). In the meantime the coil of chromatin has been 

 unraveled and has separated into a number of pieces {chromo- 

 somes), which is defiinite for a given species. These often form 

 into V-shaped loops. After certain evolutions, these loops 

 come to lie in the equatorial plane of the spindle, the apices 

 of the loops pointing toward the centre of the nucleus and the 

 threads of the spindle attached to the chromosomes. This is 

 callpd the astroid stage (Fig. 8, C). The process up to this 

 point is known as the prophase or preparation stages. 



4. Each of the chromatin loops next splits longitudinally 

 into two. This is the metaphase or middle stage (Fig. 8, D). 



5. Each of these halves now begins to move toward its 

 appropriate pole or centrosome (Fig. 8, £). As these half -loops 

 leave the equator and collect about the poles they give rise to 

 a double-star appearance or diastroid stage (Fig. 8, F). This 

 is the anaphase. 



6. The loops of chromatin collected at each pole are re- 

 constructed into a coil which then passes gradually into the 

 resting stage at the new position, a membrane is formed, and 

 the daughter nucleus is complete. The spindle disappears, the 



