MITOSIS AND AMITOSIS 13 



Prophase. — 1. The centrosome 'divides and the two minute bodies resulting 

 from the division move apart, ultimately occupying positions at opposite poles 

 of the nucleus (I-III). 



2. Astral rays appear in the cytoplasm about each centriole. They radiate 

 from it and the threads of the central or achromatic spindle are formed between 

 the two asters, thus constituting the amphiaster (II). 



3. The nuclear membrane and nucleolus disappear, the nucleoplasm and 

 cytoplasm becoming continuous. 



4. During the above changes the chromatic network of the resting nucleus 

 resolves itself into a skein or spireme, which soon shortens and breaks up into 

 distinct, heavily-staining bodies, the chromosomes (II, III). A definite number 

 of chromosomes is always found in the cells of a given species. The chromosomes 

 may be block-shaped, rod-shaped, or bent in the form of a U. 



5. The chromosomes arrange themselves in the equatorial plane of the central 

 spindle (IV). If U-shaped, the base of each U is directed toward a common center. 

 The amphiaster and the chromosomes together constitute a mitotic figure and at 

 the end of the prophase this is called a monaster. 



Metaphase.^The longitudinal spUtting of the chromosomes into exactly 

 similar halves constitutes the metaphase (IV, V). The aim of mitosis is thus ac- 

 complished, an accurate division of the chromatin between the nuclei of the 

 daughter cells. 



Anaphase. — ^At this stage the two groups of daughter chromosomes separate 

 and move up along the central spindle fibers, each toward one of the two asters. 

 Hence this is called the diaster stage (V, VI). At this stage, the centrioles may 

 each divide in preparation ioi the next division of the daughter cells. 



Telophase. — 1. The daughter chromosomes resolve themselves into a retic- 

 ulum and daughter nuclei are formed (VII, VIII). 



2. The cytoplasm divides in a plane perpendicular to the axis of the mitotic 

 spindle (VIII). Two complete daughter cells have thus arisen from the mother 

 cell. 



The compHcated processes of mitosis, by which cell division is brought about 

 normally, seem to serve the purpose of accurately dividing the chromatic sub- 

 stance of the nucleus in such a way that the chromatin of each daughter cell may 

 be the same quahtatively and quantitatively. 



This is important if we assume that the chromatic particles of the chromosomes bear the 

 hereditary qualities of the cell. The number of chromosomes is constant in the sexual ceUs 

 of a given species. The smallest number of chromosomes, two, occurs in Ascaris megalo- 



