MITOSIS 



Reproduction of the Cell 



All living organisms are built up of minute units called cells. 

 The power of an organism to reproduce itself is dependent upon 

 the ability of these protoplasmic units to divide and thereby re- 

 produce other cells. Mitosis is the name applied to the long se- 

 ries of intricately correlated changes which the nucleus under- 

 goes during the division of the cell. Cell division is especially 

 active and conspicuous in the early development of an individual 

 from a fertilized egg. The structures involved in mitosis are so 

 minute that they can not be studied easily in entire cells, con- 

 sequently cells undergoing mitosis are preserved and cut into 

 thin slices (sections) by the use of an instrument called the mi- 

 crotome. These sections are then treated with stains or dyes. 

 Various parts within the cell react differently to the stain and 

 for that reason are easily distinguishable. Chromatin, one of the 

 materials within the nucleus, is especially deeply colored by the 

 stains frequently used. 



In interpreting sections of cells in this study keep the fol- 

 lowing facts in mind : 



1. The nucleus is much smaller than the entire cell, conse- 

 quently many slices through a given cell will contain no part 

 of the nucleus. 



2. Before the mitosis has begun the nucleus is very con- 

 spicuous as a large light colored body in which some darker 

 granules of chromatin are found. 



3. Early in the process of mitosis the membrane surround- 

 ing the nucleus disappears, allowing the nuclear material to lie 

 directly in the protoplasm. From this time there is no sharply 

 defined light colored body to represent the nucleus. 



4. The spindle formed by the centrosome has but one chief 

 axis. The developing eggs are too small to be placed all in a 

 uniform position, consequently when sections are cut only part 

 of them will pass through the spindle and of these only a very 



52 



