The Cell 19 



red division or karyokinesis [Fig. 6]. This is the more usual form of 

 cell division in multicellular organisms, and is rather complicated. 

 In contrast with mitotic division the first two forms described above are 

 called amitotic. Budding is never found in cells of the higher order of 

 organisms, and fission occurs in these organisms only where the cells are 

 pathological, or are approaching the end of their lines of descent from 

 natural causes, as is the case with the cells of the membrane which lines 

 the bladder, where the cells are constantly being lost from the surface 

 and replaced by others formed below. 



In mitosis, the nucleus takes up a position near the center of the cell, 

 and the chromatin forms a relatively thick thread, usually continuous in 

 the early stages of mitosis. This chromatin thread, the skein or spireme, 

 divides longitudinally into two nearly equal threads, and each of these 

 halves next breaks up into a number .of short pieces which are called 

 chromosomes. Meanwhile the centrosome, if not double at the begin- 

 ning, has divided, and the two centrosomes have moved apart to positions 

 on opposite sides of the nucleus. The linin network at the same time has 

 been replaced by the mitotic spindle of fine lines spreading out in cone 

 shape from the centrosomes and meeting midway between. (This is a 

 typical order of events: in many cases the sequence is different. For ex- 

 ample: the lateral division of the spireme may occur before the longitu- 

 dinal division : or there may be no spireme formed. ) 



Eventually, half of the total number of chromosomes are drawn to each 

 centrosome, where they unite to form a new skein, from which the chro- 

 matin is then distributed into its usual form in the new nucleus. 



During the formation of the new nuclei the parent cell has begun to 

 constrict about the equator defined by the polar axis through the two cen- 

 trosomes. With the final completion of this constriction, the original 

 parent cell is replaced by two daughter cells. The whole process of mitotic 

 division may require from a few minutes to several hours. 



The presence of a centrosome can not be demonstrated in all cells. In 

 some cells, on the other hand, there are many centrosomes. It is possible 

 that in amitotic division the centrosome plays an important part; and 

 some investigators believe that it has an important role apart from its 

 function in reproduction. Certain cells, such as spermatozoa, some uni- 

 cellular plants and animals [Fig. 2], and the cells lining the respiratory 

 passages, have fine cilia, projecting externally, which are capable of a 

 whip-like motion. In some cells, it is quite clear that these are connected 

 with centrosomes, and the analogy between the cilia and the lines of the 

 mitotic spindle has suggested that the formation of such fibers is in every 

 case the work of centrosomes. 



