THE ANIMAL CELL. 23 



nucleus and cell merely constrict into two nearly equal parts 

 (Fig. 6) ; and (6) 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 out- 

 lined in the text and figures which follow. The nucleus will be seen to 

 be especially active 



1. In the quiescent or resting stage the structural elements are dis- 

 tributed in the way characteristic of the particular cell under examina- 

 tion (Fig. 7, A}. 



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. 7, 5). The nuclear membrane often disappears at this time. 



3. The centrosome divides and the halves migrate to opposite, 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. 7, 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. 7, C). The process up 

 to this point is known as the prophasc or preparation stages. 



4. Each of the chromatin loops next splits longitudinally into two. 

 This is the metaphase or middle stage (Fig. 7, D). 



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

 pole or centrosome (Fig. 7, ). As these half-loops leave the equator and 

 collect about the poles they give rise to a double-star appearance or dias- 

 troid stage (Fig. 7, F). This is the anaphase. 



6. The loops of chromatin collected at each pole are reconstructed into 

 a coil which then passes into the resting stage at the new position, a 

 membrane is formed, and the daughter nucleus is complete. The nuclear 

 spindle disappears, the radial appearance about the centrosomes, and even 

 the centrosome itself, may disappear or become inconspicuous. 



7. Accompanying or following the last nuclear changes the cytoplasm 

 may have become constricted into two masses, or separated by the formation 

 of a wall perpendicular to the axis of the spindle (Fig. 7, G, H}. The 

 daughter cells may separate or remain united. These final stages are known 

 as the telophase. Cell division is at the beginning of all the complexities 

 of structure found in the higher forms of animals. Each sexually produced 

 organism commences life as a single cell, from which the adult is formed 

 by cell-division, and the clinging together of the daughter cells. 



