242 ANIMAL BIOLOGY 



And now the stage is set for what is apparently the climax of 

 mitosis, designated the metaphase. Each of the chromosomes sep- 

 arates into two parts along the line of the longitudinal split already 

 present, in such a manner that each of the thousands of chromatin 

 granules which make up a chromosome is equally divided. Two 

 sets of similar daughter chromosomes are thus formed. 



With chromosomal division consummated, the metaphase 

 merges into the anaphase which is devoted to a shifting of a 

 daughter set of chromosomes along the fibers to either end of the 

 spindle. In this way each centrosome becomes associated with 

 one set of daughter chromosomes. 



The last stage, or telophase, is one of nuclear reconstruction and 

 division of the cytoplasm. The chromosomes become indistinct 

 as they spin out to form the net-like appearance of the chromatin 

 in the nucleus of each daughter cell; a nuclear membrane arises; 

 and the nucleus again assumes the form of a definite spherical 

 body characteristic of the resting cell. It must be emphasized, 

 however, that although the chromosomes usually disappear from 

 view as definitive entities in the resting nucleus, nevertheless the 

 individuality of each persists and the same chromosomes emerge 

 from the nuclear complex at the next division period. 



Simultaneously with these nuclear changes, and before the 

 spindle and asters — the machinery of mitosis — disappear, the 

 division of the cytoplasm is initiated as indicated by an indenta- 

 tion of the cell wall, encircling the cell at the equator. This be- 

 comes deeper as it gradually extends through the cytoplasm in the 

 same plane which the equatorial plate formerly occupied, until 

 the cytoplasm is cut into two separate masses, each containing 

 one of the daughter nuclei and centrosomes. Thus one cell has 

 merged its individuality into two daughter cells by mitotic divi- 

 sion. Cell division — reproduction — has occurred. 



What is the main thought that we carry away from this brief 

 view of a phenomenon that has been going on for untold ages; 

 is going on in various cells of our own bodies this very instant? 

 Surely it seems that whereas the mitotic process apparently re- 

 sults in merely a mass division of the cytoplasm, the chromatin 

 material is rearranged and distributed in a manner which makes 

 it possible for each cell to receive a very definite share. Each 

 daughter cell receives the same number of chromosomes, although 

 in many cases there is a very great difference in the size of the 



