CELL-DIVISION. EAR 70KINESIS. 



83 



the cells to which thej give rise. The twu masses of iiiesohlastic 

 cells gradually increase in size and finally fill the segmentation- 

 cavity. 



The internal phenomena of cell-division are of great complexity and 

 can here be given only in outline. The ordinary type of cell-division, as 

 shown in the segmentation of the ovum and in the multiplication of most 

 tissue-cells, involves a complicated series of changes in the nucleus known 

 as karyokinesis or mitosis. These changes, which appear to be of essen- 

 tially the same character in nearly all kinds of cells, and both in jjlants and 

 in animals, are illustrated by the following diagrams : 



C D 



Fig. 37.— Diagrams of indirect cell-division or karyokinesis. 



A. Cell just prior to division, showing nucleus (/j) with its chromatic reticulum and 

 the attraction-sphere and centrosome (c). 



B. First phase; the attraction-sphere has divided into two, which have moved 

 180° apart; the reticulum has been resolved into five chromosomes (black), each 

 of which has split lengthwise. 



C. Second phase; fully developed karyokinetic figure (amphiastcr), with spindle 

 and asters; the chromosome-halves are moving apart. 



D. Final phase; the cell-body is dividing, the spindle disappearing, tlie dauglitor- 

 nuclei about to be formed. 



In its resting state the nucleus contains a network or reticulum of 

 chromatin (Fig. 37, A), As the cell prepares for division a small body (c) 



