182 THE CELL 



which are produced by the re-arrangement of the nuclein, and 

 which constitute the chromatin portion of the figure. 



All the individual constituent portions of the division-figure as 

 a whole vary according to fixed laws, by grouping their elements 

 in various ways during the course of the process of division. 



For the sake of convenience it is well to distinguish four 

 different phases, which succeed each other in regular sequence. 



During the first stage the resting nucleus undergoes changes 

 preparatory to division, resulting in the formation of the nuclear 

 segments and the nuclear centrosomes, whilst at the same time 

 the spindle commences to develop. During the second stage the 

 nuclear segments, after the nuclear membrane has become dis- 

 solved, arrange themselves into a regular figure, midway between 

 the two poles, at the equator of the spindle. During the third the 

 daughter-segments, into which during one of the former stages the 

 mother-segments have divided by longitudinal fission, separate 

 into two groups, which travel in opposite directions from the 

 equator until they reach the neighbourhood of the centrosomes. 

 During the fourth stage reconstruction takes place, vesicular 

 resting daughter nuclei being formed out of the two groups of 

 daughter-segments, whilst the cell body divides into two daughter- 

 cells. In the next few sections a more minute description will be 

 given of the process of cell division as it occurs in some individual 

 cases, and finally a special section will be devoted to the discussion 

 in detail of certain disputed points. 



The most convenient, and at the same time the commonest, sub- 

 jects for examination in the animal kingdom are the tissue cells of 

 young larvae of Salamandra maculata, of Triton, the spermatozoa 

 of mature animals, the segmentation spheres of small transparent 

 eggs, especially of Nematodes (Ascaris megalocephala), and of 

 Echinoderms (Toxopneustes lividus). Amongst plants the proto- 

 plasm of the endosperm of the embryo sac, especially of Fritil- 

 liria imperialis, and the developing pollen cells of Liliacese, are 

 especially to be recommended. 



a. Cell division, as it occurs in the Salamandra 

 maculata, as an example of the division of the sperm- 

 mother-cell. 



First Stage. Preparation of the Nucleus for Division. 

 In the Salamandra maculata certain preliminary changes occur 

 in the resting nucleus some time before division actually com- 



