18 INTRODUCTION 



in their reactions to stains. Cytoplasm takes acid stains, there- 

 fore staining red with eosin. The chromatin of the nucleus 

 reacts mainly to basic stains so that when hematoxylin is em])loyed 

 the chromatin has a color varying from lilue to purplish-hlack. 

 In the resting nucleus the chromosomes are not apparent. Their 

 disappearance is presumably due to the incorporation of liasic pro- 

 teins in the nucleic acid molecule, which then goes into solution. 

 Two types of chromatin are thus indicated in preparations of cells 

 in the resting state; oxychromatin, which has an affinity for acid 

 dyes; and basic chromatin with an affinity for basic dyes. 



Within the nucleus of the resting cell there is usually present 

 at least one sm^iU spherical body, the nucleolus, that in living cells 

 is highly refractive. This body has an affinity for acid dyes, but 

 may also stain with basic dyes and resemble chromatin in aj^pear- 

 ance. Its function is not definitely known, but there are some 

 indications that it may be associated with the storage of nuclear 

 wastes and of chromatin. It usually disappears during the mitotic 

 process. 



The nucleus is regarded as being the regulator of cellular activities, 

 and changes in its appearance and reactions are associated with the 

 changing states of the cell as a whole. One of the major functions 

 in which the nucleus is intimately involved is cellular division, or 

 reproduction. 



Cell Reproduction.— The formation of new cells is most active in 

 the embryo, yet the process takes place in many tissues of the adult 

 vertebrate body notwithstanding the fact that there are surprisingly 

 few cells in the active process of division to be seen in the usual 

 microscopic preparations. Probably most of the cells of which 

 adult tissues are composed have a longer life than one would at first 

 suppose. We should think of a cell as a com])licated biochemico- 

 physical system whose organization is maintained for a long time, 

 even though the materials of the system may change. 



Two types of cell division have been recognized, mitosis and 

 amitosis. Mitosis, or indirect cell division, is the normal method 

 of cell division, and a brief presentation of this process may serve 

 to recall the major features. 



Mitosis.— The mitotic process is usually ari)itrarily (li\idc(j into 

 four stages: namely, prophase, metaphase, anaphase, and telophase. 

 These imperceptibly grade into one another, since they are but 

 easily distinguishable steps in a continuous sequence of phenomena. 



