CELL-DIVISION. 7 



tial is this substance for the perpetuation of the specific character of the cell, 

 that the entire complex mitotic cycle has for its primary purpose the insur- 

 ance of the equal division of the chromatin of the mother cell between the 

 two new nuclei. Such impartial distribution of the maternal chromatin takes 

 place irrespective of any, or even very great, dissimilarity in the size of the 

 daughter cells, the smaller receiving one- half, or exactly the same amount 

 of chromatin as the larger. 



Mitotic Division. The details of mitosis or karyokinesis include a 

 series of changes involving the nucleus, the centrosome and the cytoplasm. 

 These changes are grouped conveniently into four stages: (i) the Pro- 

 phases, or preparatory changes; (2) the Metaphase, during which the mass 

 of maternal chromatin is equally divided; (3) the Anaphases, in which the 

 chromatin is distributed to the new nuclei; and (4) the Telophases, during 



jsms 



FIG. 6. Chromatic figures in dividing cells from epidermis of salamander embryo. X 850. A, rest- 

 ing stage; fl, close spireme ; C, loose spireme; /?, chromosomes ("wreath"), seen from surface; E, 

 similar stage, seen in profile ; f, longitudinal cleavage of chromosomes ; G, beginning migration of seg- 

 ments towards centrosomes ; H, separating groups of daughter segments ; /, daughter groups attracted 

 towards poles of new nuclei, cytoplasm exhibits beginning cleavage. 



which the cytoplasm of the mother cell undergoes division and the daughter 

 cells are completed. 



Mitosis includes two distinct but closely associated and coincident series 

 of phenomena, the one involving the chromatin and the other the centro- 

 some of the spindle. While as a matter of convenience these two sets of 

 changes are described separately, it must be understood that they take place 

 simultaneously and in coordination. The purpose of the changes affecting 

 the chromatin is the accurate and equal division of this substance by the 

 longitudinal cleavage of the chromatin segments. The object of the activity 

 of the centrosomes and the spindle is to supply the requisite energy and guid- 



