24 ZOOLOGY 



nucleus and cell merely constrict into two nearly equal parts 

 (Fig. 7); and (b) indirect or mitotic division. The latter is 

 the usual method and is very complicated. By means of it a 

 very even division of the substances and structures of the 

 nucleus, especially, seems to be secured. 



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 wonderful 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 characteristic of the particular cell 

 under examination (Fig. 8, A) . Most cells are 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 to op- 

 posite poles of the nucleus, and from them as centres radiations 

 pass into the cell body in all directions. Across the nucleus, 

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

 producing the appearance of a spindle (Fig. 8, C, sp). In 

 the meantime the coil of chromatin has been unraveled and has 

 broken up into a definite number of pieces (chromosomes) 

 which often form into V-shaped loops. After certain evolutions, 

 under the influence of the centrospheres apparently, these loops 

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

 of the loops pointing toward the centre of the nucleus. This 

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

 point is known as the pro phase 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, E). 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. 



