70 Introductory Biology 



tion of the cell wall as in the mitosis of animal cells (contrast with Fig. 

 20). The two daughter cells continue to reform their missing parts and 

 increase in size (Fig. 22, 10). 



IMPORTANT FACTS REGARDING MITOSIS 



In the process of mitosis new cells always arise from "parent" cells. 

 The mechanical aspects of mitosis are remarkable, complicated, and at 

 present not understood completely. The nucleus undergoes the more 

 visible changes during mitosis. These changes are in all probability 

 both chemical and physical. Undoubtedly the surrounding cytoplasm 

 also plays an important role in mitosis, although the exact nature of it 

 is at present unknown. At certain stages the nucleoplasm and cytoplasm 

 are not separated by the usual nuclear membrane. 



During the entire process of mitosis, there is a continuity of chromatin 

 in some form or other from the original parent cell to the two daughter 

 cells. The chromatin is divided accurately and equally between the 

 newly formed cells. Chromatin transmits hereditary genes from one 

 cell to others, and also from the parents of one generation to the off- 

 spring of the next or following generations. In this way chromatin ma- 

 terials are responsible for the inheritance of certain characteristics which 

 are passed from one cell to the next and at the proper time will deter- 

 mine that particular characteristic in the living organism. 



The first signs of mitosis (a) in animal cells are the division and 

 migration of the centrosomes and (b) in plant cells, the formation of 

 chromosomes from the strands of chromatin granules. Later, in the 

 telophase stages, the cell membrane of animal cells indents to form the 

 cell membrane between the daughter cells; the cell wall between daugh- 

 ter cells of plants is formed by an accumulation of granules along the 

 equatorial plate without an indentation of the original cell wall (Figs. 

 20 and 22). 



Each species of plant or animal has a definite number of chromosomes 

 which appear when the cells of that particular organism divide. Most 

 animals and plants have an even number of chromosomes (occur in 

 pairs), although a few species have an odd number in their cells. (For 

 a more complete table showing the numbers of chromosomes in cells, 

 see the chapter on Heredity.) 



Mitosis plays an important role in growth. Living organisms grow 

 either by an increase in the number of cells (by mitosis) or by an in- 

 crease in the size of cells already present. In many cases a combination 

 of these two methods results in the growth of the organism. The forma- 



