STRUCTURE AND ACTIVITIES OF CELLS 



division of the nucleus, which may occur by the method of amitosis or by 

 mitosis. 



Amitosis. In amitosis, or direct nuclear division, the nucleus becomes 

 somewhat elongated and constricts into two parts which are about equal in 

 volume. The nuclei of certain types of cells may divide amitotically without 

 division of their cytosomes and thus give rise to multinucleate cells. How- 

 ever, the cytosome may divide after the nucleus is constricted, and two new 

 cells are formed. The distribution of nuclear components is only approxi- 

 mately equal in this direct process of division. Such a type of division 

 apparently occurs most often in cells that are very specialized, very old, or 

 in some abnormal or degenerating condition. 



Mitosis. The typical method of nuclear division is by mitosis. It is called 

 the indirect method because it involves changes that are more complicated 

 than the simple constriction of amitosis. The process of mitosis, which was 

 first fully studied in animal cells by Walter Flemming in 1878, is divided for 

 purposes of description into four continuous stages: prophase, metaphase, 

 anaphase, and telophase. 



The general structure of a vegetative cell should be recalled, with particular 

 reference to the cell center and the nucleus (p. 21). In some cells there 

 are two centrioles during the vegetative phase; in others, only one. For this 

 account let us consider a cell in which two centrioles are present. Among 

 the earliest changes to occur in the prophase of mitosis is the separation of 

 the two centrioles toward opposite sides of the nucleus (Fig. 2.10). At the 

 same time delicate fibers become visible about the centrioles in the region of 

 the cell center. The fibers that stretch between the centrioles as they move 

 apart are known as the spindle fibers, since they converge toward the centrioles 

 in a typical spindle formation. Fibers called astral rays extend freely from 

 each centriole into the surrounding cytoplasm. The structure formed by the 

 fibers and the centrioles is known as the mitotic spindle, because of the arrange- 

 ment of the fibers that pass from one centriole to the other. The source of the 

 fibers and their exact nature are not clear, but the reality of the mitotic 

 spindle is indisputable (Fig. 2.1 B); by suitable methods it can be isolated 

 from the cell. While the mitotic spindle is being formed in the cytosome, 

 delicate chromatic threads appear in the nucleus and are seen to be double; 

 that is, two threads are found close together. Around these two genonemata, 

 so-called because they are apparently made up of the linearly arranged hered- 

 itary units or genes (p. 190), intensely staining chromatic material accumu- 

 lates. These elements shorten and thicken forming chromosomes (Fig. 2.10). 

 Chemically the chromosomes are known to contain desoxyribonucleic acid 

 (DNA) and characteristic proteins. When chromosomes are gently disso- 

 ciated, they are demonstrated to be composed of macromolecular granules 

 which may be the genes, or clusters of them, held together by ionic bonds 

 (p. 23). 



Chromosomes may be different in size and shape in the cells of an organ- 

 ism; that is, round chromosomes and straight and bent rods occur. The shape 



39 



